But if we walk in the light as He is in the light, we have fellowship with one another

1 John 1:17
But 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.

Whatever the sin is, big or small, it is untouchable to the Holy God. God hates sin, so as we learn to walk with Him, we too should hate sin. It is more than just doing wrong or morally below standard. It also has something to do with our relationship with God, whether we obey and follow Him, or if we ignore Him. This is something the Holy Spirit can point out to us as we walk with God. So this means at the time of becoming a Christian, we may still not fully understand what sin is, but it is the beginning of a journey of being led by the Spirit.