Well, from where do you think Christians came up with it? Unlike some dogmas of the church, the triune nature of Almighty God was not something made up or compromised on to please everyone. Though the word "trinity" is not used in the Bible, there is no getting around the fact that the idea is all throughout it and the number THREE comes up when the nature of God is mentioned. From the source http://www.layevangelism.com/qreference/chapter10.htm:
The Three-fold Benediction of Numbers 6:24-26 is similar to Paul’s Benediction of 2 Corinthians 13:14. Paul had to have in mind Numbers 6:22-27 when he gave this benediction. Numbers 6:22-27 "Then Yehovah spoke to Moses, saying, ‘Speak to Aaron and to his sons, saying, "Thus you shall bless the sons of Israel. You shall say to them: Yehovah bless you, and keep you; Yehovah make His face shine on you, and be gracious to you; Yehovah lift up His countenance on you, and give you peace." So they shall invoke My name on the sons of Israel, and I then will bless them.’" (Deut 6:22-27)
2 Corinthians 13:14 "The grace of the Lord Jesus Christ, and the love of God, and the fellowship of the Holy Spirit, be with you all."
Jesus had this in mind when He commanded that we be baptized in the name of the Trinity:
Matthew 28:18-20 "And Jesus came up and spoke to them, saying, ‘All authority has been given to Me in heaven and on earth. GO THEREFORE and make disciples of all the nations, baptizing them in the name of the Father and the Son and the Holy Spirit, teaching them to observe all that I commanded you, and lo, I am with you always, even to the end of the age.’"
The Trinity in Scripture:
Isaiah 48:16 "Come near to Me, listen to this: from the first I have not spoken in secret, from the time it took place, I was there, and now Adonai-Yehovah has sent Me and His Spirit." [Indication of the Son and the Holy Spirit being a person & the Trinity] (John 18:20, Jesus said this referred to Him; John 13:19, 14:29)
Matthew 3:16-17 "And after being baptized, Jesus went up immediately from the water; and behold, the heavens were opened, and he saw the Spirit of God descending as a dove, and coming upon Him, and behold, a voice out of the heavens, saying, ‘This is My beloved Son, in whom I am well-pleased.’" (Luke 3:21-22)
Matthew 28:18-20 "And Jesus came up and spoke to them, saying, ‘All authority has been given to Me in heaven and on earth. go therefore and make disciples of all the nations, baptizing them in the name of the Father and the Son and the Holy Spirit, teaching them to observe all that I commanded you, and lo, I am with you always, even to the end of the age.’"
Luke 3:21-22 "Now it came about when all the people were baptized, that Jesus also was baptized, and while He was praying, heaven was opened, and the Holy Spirit descended upon Him in bodily form like a dove, and a voice came out of heaven, ‘You are My beloved Son, in You I am well-pleased.’" (Matt 3:16-17, Isaiah 48:16)
John 1:32-34 "And John gave testimony saying, ‘I beheld the Spirit descending as a dove out of heaven and He remained upon Him. And I did not know Him, but the one who sent me to baptize in water, that One said to me, ‘On whom you see the Spirit coming down upon and remaining, this is the one baptizing in the Holy Spirit.’ And I have seen, and witnessed that this is the Son of God."
John 14:26 "But the Helper, the Holy Spirit, whom the Father will send in My name, He will teach you all things, and bring to your remembrance all that I said to you."
John 15:26 "When the Helper comes, whom I will send to you from the Father, that is the Spirit of truth, who proceeds from the Father, he will bear witness of Me."
John 16:13-15 "But when He, the Spirit of truth, comes, he will guide you into all the truth; for He will not speak on His own initiative, but whatever He hears, He will speak; and He will disclose to you what is to come. He shall glorify Me; for He shall take of Mine, and shall disclose it to you. All things that the Father has are Mine; therefore I said, that He takes of Mine, and will disclose it to you."
1 Corinthians 12:4-6 "Now there are varieties of gifts, but the same Spirit. And there are varieties of ministries, and the same Lord. And there are varieties of effects, but the same God who works all things in all persons."
Galatians 4:6 "And because you are sons, God has sent forth the Spirit of His Son into our hearts, crying, "Abba! Father!"
Ephesians 2:18 "For through Him we both have our access in one Spirit to the Father."
Ephesians 4:4-6 "There is one body and one Spirit, just as also you were called in one hope of your calling; one Lord, one faith, one baptism, one God and Father of all who is over all and through all and in all."
1 Thessalonians 5:23 "Now may the God of peace Himself sanctify you entirely." (The Father Sanctifies us)
1 Corinthians 1:30 "But by His doing you are in Christ Jesus, who became to us wisdom from God, righteousness and sanctification, and redemption" (Jesus Sanctifies us)
Hebrews 2:11 "For both He who sanctifies and those who are sanctified are all from one Father; for which reason He is not ashamed to call them brethren."
Hebrews 10:10 "By this will we have been sanctified through the offering of the body of Jesus Christ once for all."
1 Peter 1:2 "According to the foreknowledge of God the Father, by the sanctifying work of the Spirit, that you may obey Jesus Christ and be sprinkled with His blood" (The Holy Spirit Sanctifies us)
Genesis 2:7 "Then Yehovah Elohim formed man of dust from the-ground, and breathed into his-nostrils the breath of lives; and man became-a-living soul." (The Father gives life)
John 1:3 "All things came into being by Him, and apart from Him nothing came into being that has come into being." (Jesus gives life)
John 5:21 "For just as the Father raises the dead and gives them life, even so the Son also gives life to whom He wishes."
John 6:63 "It is the Spirit who creates-life, the flesh profits nothing."
2 Corinthians 3:5-6 "Not that we are adequate in ourselves to consider anything as coming from ourselves, but our adequacy is from God, who also made us adequate as servants of a new covenant, not of the letter, but of the Spirit; for the letter kills, but the Spirit creates-life." (The Holy Spirit gives life)
1 John 1:3 "What we have seen and heard we proclaim to you also, that you also may have fellowship with us; and indeed our fellowship is with the Father, and with His Son Jesus Christ." (We have fellowship with the Father & the Son)
Corinthians 1:9 "God is faithful, through whom you were called into fellowship with His Son, Jesus Christ our Lord." (We have Fellowship with Jesus)
2 Corinthians 13:14 "The grace of the Lord Jesus Christ, and the love of God, and the fellowship of the Holy Spirit, be with you all." (We have Fellowship with the Holy Spirit)
Psalm 90:2 "Before the mountains were born, or You did give birth to the earth and the world, even from everlasting to everlasting, You are El." (The Father is from eternity)
Micah 5:2 "But as for you, Bethlehem Ephrathah, too little to be among the clans of Judah, from you One will go forth for Me to be ruler in Israel. His goings forth are from long ago, from the days of eternity." (Jesus is from eternity)
John 8:58 "Jesus said to them, ‘Truly, truly, I say to you, before Abraham was born I AM.’"
Hebrews 9:14 "How much more will the blood of Christ, who through the eternal Spirit offered Himself without blemish to God, cleanse your conscience from dead works to serve the living God?" (The Holy Spirit is from eternity)
Luke 22:42 "Father, if You are willing, remove this cup from Me; yet not My will, but Your’s be done." (The Father & the Son have a will)
1 Corinthians 12:11 "But one and the same Spirit works all these things, distributing to each one individually just as He wills." (The Holy Spirit has a will)
Matthew 3:17 "And behold, a voice out of the heavens, saying, ‘This is My beloved Son, in whom I am well-pleased.’" (The Father speaks)
Luke 5:20 "And seeing their faith, He said, ‘Friend, your sins are forgiven you.’" (Jesus speaks)
Luke 7:48 "And He said to her, ‘Your sins have been forgiven.’"
Acts 8:29 "And the Spirit said to Philip, ‘Go up and join this chariot.’" (The Holy Spirit speaks)
Acts 11:12 "And the Spirit told me to go with them without misgivings."
Acts 13:2 "And while they were ministering to the Lord and fasting, the Holy Spirit said, ‘Set apart for Me Barnabas and Saul for the work to which I have called them.’"
John 3:16 "For God so loved the world, that He gave His only begotten Son, that whoever believes in Him should not perish, but have eternal life." (The Father loves us)
Ephesians 5:25 "Husbands, love your wives, just as Christ also loved the church and gave Himself up for her." (Jesus loves us)
Romans 15:30 "Now I urge You, brethren, by our Lord Jesus Christ and by the love of the Spirit, to strive together with me in your prayers to God for me." (The Holy Spirit loves us)
Ephesians 5:18-20 "And do not get drunk with wine, for that is dissipation, but be filled with the Spirit, speaking to one another in psalms and hymns and spiritual songs, singing and making melody with your heart to the Lord; always giving thanks for all things in the name of our Lord Jesus Christ to God, even the Father."
1 Peter 1:2 "According to the foreknowledge of God the Father, by the sanctifying work of the Spirit, that you may obey Jesus Christ and be sprinkled with His blood: May grace and peace be yours in fullest measure."
1 John 5:7 "For there are three that bear record in heaven, the Father, the Word, and the Holy Spirit; and these three are one."
Jude 20-21 "But you, beloved, building yourselves up on your most holy faith; praying in the Holy Spirit; keep yourselves in the love of God, waiting anxiously for the mercy of our Lord Jesus Christ to eternal life."
Hebrews 1:8 "But of the Son He says, ‘Your throne, O God [Elohim, !yhiloa>], is forever and ever, and the righteous scepter is the scepter of Your kingdom.’"
Psalm 45:6 "Your throne, O Elohim, is forever and ever, a scepter of uprightness is the scepter of your kingdom." (Jesus is Elohim)
2 Timothy 1:2 "Grace, mercy and peace from God the Father and Christ Jesus our Lord." (The Father is God)
Philippians 2:10-11 "that at the name of Jesus every knee should bow, of those who are in heaven, and on earth, and under the earth, and that every tongue should confess that Jesus Christ is aAdonai, to the glory of God the Father." (From Isaiah 45:21-23. Jesus is Yehovah)
Isaiah 45:21-23 "Declare and set forth your case; indeed, let them consult together. Who has announced this from of old? Who has long since declared it? Is it not I, Yehovah? And there is no other Elohim besides Me, a righteous El and a Savior; there is none except Me. Turn to Me, and be saved, all the ends of the earth; for I am El, and there is no other. I have sworn by Myself, the word has gone forth from My mouth in righteousness and will not turn back, that to Me every knee will bow, every tongue will swear allegiance." (Phil 2:10-11, Jesus is Yehovah)
John 1:1 "In the beginning was the Word, and the Word was with God and the Word was God [qeo;" h\\n oJ logo"]. And the Word became flesh, and dwelt among us." (Jesus is God)
John 1:18 "No man has seen God at any time; the only begotten God, who is in the bosom of the Father, He has explained Him." (This means that the appearances of Yehovah God in the O.T. are not the Father, but Jesus-Yehovah, Genesis 18:1, because no one has ever seen the Father)
Genesis 18:1 "Now Yehovah appeared to him by the oaks of Mamre, while he was sitting at the tent door in the heat of the day." (Jesus appeared to him, John 1:18)
Acts 5:3-4 "But Peter said, ‘Ananias, why has Satan filled your heart to lie to the Holy Spirit, and to keep back some of the price of the land? While it remained unsold, did it not remain your own? And after it was sold, was it not under your control? Why is it that you have conceived this deed in your heart? You have not lied to men, but to God.’" (The Holy Spirit is God)
Moses wrote in Genesis 1:1 "In-the-beginning Elohim created the heavens and the earth." The Term Elohim is the general term for "God" in Hebrew. Elohim was also the universal term used for "God" in many of the surrounding cultures of Moses’ time. Elohim, !yhiloa> (Hebrew is read from right to left), is a singular plural noun broken down into two words: lae El and !yhi heem. lae means God and !yhi means these. Together they mean These-are-God," thus indicating the Triune nature of God. This is why when God created man that God says in Genesis 1:26 "Let Us make man in Our-image, according to Our-likeness." Man Himself is a trinity of spirit, soul and body but of one essence, one human being. Paul wrote in 1 Thessalonians 5:23,
"Now may the God of peace Himself sanctify you entirely; and may your spirit (pneu'ma) and soul (yuchv) and body (sw'ma) be preserved complete, without blame at the coming of our Lord Jesus Christ." (1 Thess 5:23)
Believe me, there are MANY more verses I could give you. The point is that the Apostles got it that Jesus was God in the flesh - once they came to full faith and the Holy Spirit indwelt them. They knew that the prophecies of the Messiah said he would be "God with us". All throughout the the Gospels and Epistles, the nature of God Almighty was revealed and clarified and, because they had faith, they accepted this truth EVEN though it is humanly impossible to fully comprehend. We have finite minds and God is infinite. I don't understand how the Father, Son and Holy Spirit can be individually and independent yet STILL be ONE God but I have faith that the Bible states this is the case, so I believe it. Jesus told his disciples that he explained earthly things to them and they didn't believe him, how would they believe if he told them heavenly things?
The Bible teaches us that there is only one True God from eternity past who is a Trinity of three individual Persons with independent intellect, emotions and will: the Father & the Son & the Holy Spirit. We do not mean that there are three individual god's who make up one Godhead as is taught in some religions. There is only one God, meaning of one essence, who is a Trinity of three persons. Take any one of these persons away and you no longer have the one and only True God who created the universe and man.
I do hope you will go to the link I gave and at least read what is there. I am completely convinced that Almighty God IS triune/Trinity because HE told us all about it through Jesus' words and throughout the Bible which is from Him to us so that we could know the true God.
Thank you for this post and these verses.