What does the Bible say about spiritual struggle in Christians?

The Bible teaches that Christians undergo a spiritual struggle, often depicted as a conflict between the old nature (the flesh) and the new nature (the spirit). This internal battle is illustrated in Romans 7, where Paul expresses his frustration that while he desires to do good, he often finds himself doing the opposite. This struggle serves as a sign of life; it indicates that one has been born again and now possesses both a fleshly desire for sin and a spiritual desire to follow God. It is through this warfare that believers learn the necessity of relying on God's grace and strength for victory over sin.