Under the covenant which God made with Israel through Moses, there was a system of sacrifices which people had to make in order to deal with their failures to obey God. These sacrifices could not really take away their sins (He.10:4). Their sins would be kept covered (Ps.32:1) till Jesus would come and offer the only sinless and sufficient sacrifice that could take sins away (Jn.1:29). God also knew that no one would be able to keep all the commandments (Ro.3:20). Then why did God make this covenant?
God had to let the people try to keep the commandments and come to realise that it was impossible! But when He saw that this was done, He came out with the new covenant through Jesus.
One big mistake many people make about this new covenant is to assume that since Jesus has made a once-for-all sacrifice (He.7:27) for sins, all they have to do is to confess their sins every time they fall (1Jn.1:9), and continue as before. Yes, it is true that Jesus has made a once-for-all sacrifice, and that we can be forgiven when we confess our sins. But the mistake is to assume that we can continue to live as before.
Many Christians think of salvation only as forgiveness of sins which God has graciously given them. Once they have forgiveness for sins, their focus turns to getting a comfortable life on earth! That is really no abundant life (Jn.10:10). The truth is that God does not want to leave us in perpetual failure in sins, with Him just picking us up and washing us clean whenever we fall. Jesus has come to 'take away' sin, and to set us free from the bondage to sin (Jn.8:34).
God has no pleasure in receiving sacrifices from us, but He wants us to actually do His will in our bodies (He.10:5-7). Jesus did the Father's will entirely in His body while He was on earth, and He wants us to follow Him on this way. That is what it really means to follow Jesus (1Jn.2:6).
God has made the new covenant in such a way that now He not only tells us to do His will, but He also enables us to do it. When we are born again through our faith in Jesus, the Holy Spirit regenerates us. He writes God's laws upon our heart and mind and causes us (enables us) to walk in them (Ez.36:25-27). In this way Jesus breaks the bondage we were in to the power of sin, and makes us free to choose the will of God (Lk.4:18,19).
It is not that we will not sin once we come into this new covenant, but that now we are free to choose not to sin. If we fall, which we may all do because of our ignorance or weakness, God has made provision for our forgiveness (1Jn.2:1,2). But He wants us not to sin (v.1). As long as we are on this earth, carrying our sinful flesh, we will not become sinless or perfect. But God wants us to press on perfection at every point (He.6:1).
If we are truly born of God, we too will want to stop sinning and to press on to perfection. The Bible says that this is one way the children of God are to be distinguished from others (1Jn.3:8-10).