Why do believers still sin?

Sin is greatly confusing for believers. The apostle captures this in Romans 7 when he says “the things that I will not, these I do.” How could the apostle seem to speak in such a defeated manner with regard to sin in a believer who has been given the Spirit and the grace of repentance? And if such a double-minded “believer” could be conceived of in this in life, is assurance of salvation possible?
To answer these questions, some have tried to explain Romans 7 as speaking of a man before conversion since a “defeatist” view of sin in the believer seems entirely out of accord with the New Testament teaching on regeneration and holiness. This answer would seem to take away any notion that a believer might so willingly enter into sin as a new creation.
… In his great work Speculum Christianum or The Christian Survey of Conscience, Girolamo Zanchi addressed at length the Reformed view of Romans 7 as the regenerate man’s struggle against sin, a view he assures was held by all the learned divines. He makes a crucial distinction that in the regenerate is a double-man. The Christian, he says, has a fundamental quality that is different from that of the unbeliever. When a regenerate man sins, “he sins only in the flesh and not with the whole will and the whole heart.”

Why Do Believers Still Sin? | The Heidelblog
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