I will not, at this time, confront the obnoxious idea that Christians do not sin. Instead I would like to look closely (though not exhaustively) at why God still allows us to sin.
One reason is so that we can identify with the mercy and grace that will be newly bestowed on those we evangelize and God brings to the faith through His word. We may lose all connection with the power and working of sin in the world if we were perfect in the instance of our conversion.
Another reason is to defeat the less obvious sins, like pride, idolatry, lust, deceit, coveting, etc. After combating the obvious sins, we realize that the same things work more subtly in us and originate from a corrupted heart (Jeremiah 17:9).
Moreover, we remember what we are capable of and what God has saved us from. The sweetness of heaven is all the more clearly imagined and hoped for after the putrid taste of sin. Our dependence on Jesus Christ is continually highlighted when we sin. Each sin causes us to realize the hatefulness of sin. In the same degree, we realize the glory and grace of Christ Jesus our Savior. The Lordship of Jesus is also reaffirmed because we more clearly see that only God has our best interests in mind. By denying God’s will, we injure ourselves when we sin. We must acknowledge His omniscience and goodness – He knows and has prescribed only what leads to our good and His glory.
Further, we gain insight into the nature of sin as treason against God. The way sin operates is better revealed to us. Its deceptiveness and lies work relentlessly to corrupt our dependence on the Holy Spirit. Sin in our lives also builds humility in us, realizing the holiness of God and His grace in saving us from eternal punishment for these sins.
We are driven to our knees in prayer when we sin. We have to come before the One whom we sin against in confession and repentance. We have sinned against Him only (Psalm 51:4) and only He can forgive us and restore us to fellowship with Him. Our prayer must not be held off until we “feel” ready to repent. To do so would further the series of sins already committed.
In closing, I remind you that Romans 6 says that we should not sin in the hope of heightening God’s glory. Nor are we permitted to sin because we are free from law but under grace. Instead, we must abstain from sin because we now belong to Christ and no sin has power over us. We now belong to Jesus Christ and must claim His promise that we were ransomed away from our old ways by the blood of Christ (1 Peter 1:18-19) and that those old sins no longer have power over us (Romans 6:18).