There are two issues: Can you know that you're a Christian?, and Can a true Christian lose his salvation?
1. Are you a Christian? The Bible provides several tests, including:
* John 3:3: "...Jesus declared, "I tell you the truth, no one can see the kingdom of God unless he is born again.""among many others, demonstrating it is possible to know that one is indeed a true Christian.* 1 John 5:11-12a: "And this is the testimony: God has given us eternal life, and this life is in his Son. He who has the Son has life..."
* Romans 10:9: "That if you confess with your mouth, "Jesus is Lord," and believe in your heart that God raised him from the dead, you will be saved."
2. Can a true Christian lose his salvation? The Bible argues he cannot: once saved, always saved.
* John 10:27-29: "My sheep listen to my voice; I know them, and they follow me. I give them eternal life, and they shall never perish; no one can snatch them out of my hand. My Father, who has given them to me, is greater than all; no one can snatch them out of my Father's hand."
Not even you can snatch yourself away from Him.* John 5:24: ""I tell you the truth, whoever hears my word and believes him who sent me has eternal life and will not be condemned; he has crossed over from death to life."
It's past-tense: "he has crossed", he "has eternal life," which by definition is eternal and cannot be lost.* Romans 8:1-2: "Therefore, there is now no condemnation for those who are in Christ Jesus, because through Christ Jesus the law of the Spirit of life set me free from the law of sin and death."
In Christ = no condemnation
By the way, a week or two ago I offered to demonstrate this doctrine of eternal security to another Catholic on this forum, in order to encourage her, and encountered in return as much unwarranted venom and vitriol as I've ever seen; so I hope you'll receive this as being born solely out of genuine helpfulness and concern. To the extent that my presentation here is inadequate, I'm quite sure an online search into the doctrine of eternal security will prove fruitful for you. Indeed, it's a shame to be left in lifelong suspense about the answer to life's biggest question, when the Lord anticipated and forestalled that suspense by providing such clear answers in His Word.
All that said, why this preoccupation with suicide?
Scripture doesn’t agree with OSAS as stated below.
“2. Can a true Christian lose his salvation? The Bible argues he cannot: once saved, always saved.”
“no one can snatch them out of my Father’s hand”
Absolutely no power can take a believer from God’s hand, but you can renounce you salvation. With that being said your salvation wasn’t taken you gave it away. In more human terms your citizenship cannot be taken from you, but you sure can renounce it.
John 8:31 King James Version (KJV)
31 Then said Jesus to those Jews which believed on him, If ye CONTINUE in my word, then are ye my disciples indeed;
2 John 9 King James Version (KJV)
9 Whosoever transgresseth, and ABIDETH not in the doctrine of Christ, hath not God. He that ABIDETH in the doctrine of Christ, he hath both the Father and the Son.
1 John 2:24 King James Version (KJV)
24 Let that therefore abide in you, which ye have heard from the beginning. If that which ye have heard from the beginning SHALL remain in you, ye also shall CONTINUE in the Son, and in the Father.
2. It’s past-tense: “he has crossed”, he “has eternal life,” which by definition is eternal and cannot be lost.
I was saved past tense all that means is yesterday for tomorrow I could start my old life again and trust me I wouldn’t not be in Jesus nor him in me. Again it’s eternal if you continue in the repentant life that Jesus has called us to live.
3. In Christ = no condemnation
Absolutely true if you continue “IN CHRIST” as stated above.
The New Covenant requires two parties both have to uphold there part to stay in force. God will always uphold his part, but we must continue in our part also.
If you couldn’t lose you salvation then Satan would NOT tempt believers we would be a lost cause OSAS. Please don’t tell me that we would lose reward because a ghetto seat in Heaven will be better than the best seat in Hell!
Jesus was tempted also and by his own free will he resisted and by his own free will he went to the Cross. I think if you really study this Scripture you will notice that Jesus had free will to chose and if he had choice why would you think we don’t?
John 10:18 King James Version (KJV)
18 No man taketh it from me, but I lay it down of myself. I have power to lay it down, and I have power to take it again. This commandment have I received of my Father.
Lucifer was saved and lost his Salvation, so how did that work?
Bottom line: You can lose your salvation if you surrender it, but it cannot be taken from you by any power or force.