The Bible talks about how that man has fallen short of God’s Glory, through the original sin of Adam and Eve. Jesus dying on the cross filled in that gap, providing us a way back home. However, we must strive to live a sin-free life. Sure, the flesh is weak (Matthew 26:41), but we must resist the act of sinning. Jesus will forgive us of those sins we do commit, if we ask for forgiveness, but that is not a get out of jail free card.
As we see in Luke 9:23 NIV:
Then he said to them all: “Whoever wants to be my disciple must deny themselves and take up their cross daily and follow me.
Sure we will be tempted, but temptation alone is not the act of committing a sin. It is when we allow ourselves to act upon that impulse, whether in our mind or physically. To be a disciple of Jesus, one must cast off sin. It is an either or. This is why when we do commit sin and do not ask for forgiveness, it feels as if Jesus has forsaken us. It is not He who has forsaken us, but we who have forsaken Him.
This is what Jesus was talking about when He said to ‘sin no more’.