An act of sin does not cost you your salvation. There are people who teach that if you sin once after you have accepted Jesus, you must be saved again. But that is not what the Bible teaches.
Can you conceive of somebody adopting a child and then telling the child that if he walks on the floor with muddy shoes he will be thrown out in the street? We would not do that to a child, and God will not do it to us, either. When you are saved, you are adopted into the family of God. You must, out of love on one hand and godly fear on the other, try to live a life that is pleasing to Him. But the idea that one act of sin would cause someone to be thrown out of God’s family is just not in agreement with the Bible. The Bible says, “If we walk in the light as He is in the light, we have fellowship with one another, and the blood of Jesus Christ His Son cleanses us from all sin” (I John 1:7).
And we are told in the little book of I John, “If we confess our sins, He is faithful and just to forgive us our sins and to cleanse us from all unrighteousness” (I John 1:9).
Acts of sin or rebellion will take away the joy of your salvation. When David committed adultery with Bathsheba and had her husband killed, he lost the joy of his salvation. When he wrote Psalm 51 about it, he said, “Restore to me the joy of Your salvation” (Psalm 51:12). He had no joy, because he had rebelled against God. Then he said, “Do not take Your Holy Spirit from me” (Psalm 51:11). Even though he had committed adultery and had been responsible for an innocent man’s death, by his clear statement he still had the Holy Spirit. But, David did not get away without being punished. He had a rebellion in his own household and actually lost his throne for a time. Nevertheless, God forgave and loved him because David was repentant before the Lord.
If you continue in a course of known sin, you will lose the assurance of your salvation, however. You will not know if the Holy Spirit is with you because you will be at odds with God. This is not referring to an actual loss of your salvation or to the Holy Spirit, only to your awareness and assurance of salvation and the Holy Spirit. The Bible says, “Anyone born of God does not continue to sin” (I John 5:18, NIV). It does not say that a Christian never commits an act of sin, but that he does not continue in a course of sin, refusing to confess and repent of his sins. So if someone has been born of the Spirit of God, there will be something inside of him drawing him back to repentance every time he sins.
Beyond that, we can gather from reading in the book of Hebrews that if somebody actually says the blood of Jesus Christ is a despised thing and renounces the salvation he has received, then that person may have lost it all. But then the Bible immediately says, “But, beloved, we are confident of better things concerning you” (Hebrews 6:1-9). It is very hard to believe that someone who has been born again will turn that far away from God.
But you might ask, if we are new creatures in Christ, why do we even have the capacity still to sin? The answer is that perfection for the Christian awaits us in heaven: “When this corruptible has put on incorruption, and this mortal has put on immortality, then shall be brought to pass the saying that is written: ‘Death is swallowed up in victory.'” (I Corinthians 15:54)
We are united with Jesus at salvation, but we are being progressively transformed into His nature.
The Bible says, “We all, with unveiled face, beholding as in a mirror the glory of the Lord, are being transformed into the same image from glory to glory.” (II Corinthians 3:18) We are being changed throughout our lives, but there is no instantaneous moment, short of death, when the believer becomes sinlessly perfect. The apostle John said, “If we say that we have no sin, we deceive ourselves, and the truth is not in us.” (I John 1:8)