Fortunately we can know for certain of our salvation based on His word.
13In Him, you also, after listening to the message of truth, the gospel of your salvation—having also believed, you were sealed in Him with the Holy Spirit of promise, 14who is given as a pledge of our inheritance, with a view to the redemption of God’s own possession, to the praise of His glory. Ephesians 1:13-14
16“For God so loved the world, that He gave His only begotten Son, that whoever believes in Him shall not perish, but have eternal life. 17“For God did not send the Son into the world to judge the world, but that the world might be saved through Him. 18“He who believes in Him is not judged; he who does not believe has been judged already, because he has not believed in the name of the only begotten Son of God. John 3:16-18
5Who is the one who overcomes the world, but he who believes that Jesus is the Son of God? 6This is the One who came by water and blood, Jesus Christ; not with the water only, but with the water and with the blood. It is the Spirit who testifies, because the Spirit is the truth. 7For there are three that testify: 8the Spirit and the water and the blood; and the three are in agreement. 9If we receive the testimony of men, the testimony of God is greater; for the testimony of God is this, that He has testified concerning His Son. 10The one who believes in the Son of God has the testimony in himself; the one who does not believe God has made Him a liar, because he has not believed in the testimony that God has given concerning His Son. 11And the testimony is this, that God has given us eternal life, and this life is in His Son. 12He who has the Son has the life; he who does not have the Son of God does not have the life. 13These things I have written to you who believe in the name of the Son of God, so that you may know that you have eternal life 1 John 5:5-13
Regarding the issue of whether Christians have an “absolute” assurance of salvation, regardless of their actions, consider this warning Paul gave: “See then the kindness and the severity of God: severity toward those who have fallen, but God’s kindness to you, provided you continue in his kindness; otherwise you too will be cut off” (Rom. 11:22; see also Heb. 10:26–29, 2 Pet. 2:20–21).