Why do Christians still sin if they have a new nature?

Even though Christians are given a new nature that is created in righteousness and holiness, they still contend with the remnants of their old sinful nature. Romans 7:18-19 illustrates this struggle, where Paul articulates that though he desires to do good, he often finds himself doing the very evil he wishes to avoid. This internal conflict is a consequence of living in a fallen world where the old man of flesh is not eradicated upon salvation. Instead, Christians live in a state of duality, constantly battling the desires of the flesh while pushing forward in the pursuit of holiness through the power of the Spirit. This ongoing struggle underscores the necessity for continual reliance on God's grace.