When we come to Christ, we receive the forgiveness of sins because of his sacrifice on the cross, shedding his blood for us. The shed blood of Jesus is supernaturally eternal and always available, because it is continually needed throughout our lives on earth. Believing in Jesus is a day by day decision and we live amongst disobedient people being used by the enemy to try and entangle us in sin. There's no need to be afraid of the enemy. We just walk in the light as he himself is in the light and the eternal blood continually cleanses us.
1John 1:5-7 shows us this truth
This is the message we have heard from Him and announce to you, that God is Light, and in Him there is no darkness at all. If we say that we have fellowship with Him and yet walk in the darkness, we lie and do not practice the truth; but if we walk in the Light as He Himself is in the Light, we have fellowship with one another, and the blood of Jesus His Son cleanses us from all sin. - 1John 1:5-7