For those who live according to the flesh set their minds on the things of the flesh, but those who live according to the Spirit, the things of the Spirit. . . . For if you live according to the flesh you will die; but if by the Spirit you put to death the deeds of the body, you will live. (Romans 8:5, 13)

Today’s big question: why do Christians still struggle with the flesh?

When studying Christian living, we should not forget about the Holy Spirit who dwells within believers. As today’s verse states, Christians are to live in the Spirit. However, this may bring up the question, if people that live in the flesh will die, then why do Christians still struggle with their flesh?

If any Christian ever told you, “I never sin,” then that person would be sinning—or more specifically, lying. Scripture clearly states that no person is without sin (1 John 1:8). This truth is easily supported by the method of observation: there is no one perfect—other than God, of course—as we are all fallible human beings. Paul is also clear on the fact that believers still struggle with the flesh:

“For what I am doing, I do not understand. For what I will to do, that I do not practice; but what I hate, that I do.” (Romans 7:15)

We must understand what Paul means by living “according to the flesh” in order to gain insight in answering today’s question. A key fact to note is that even though Paul struggled with things he did not want to do, he still hated those things. As believers, we now have the Holy Spirit living within us, and our passions are different than before. If someone lives “according to the flesh,” then that will be their passion and desire.

Furthermore, the work of Christ made it so that the law no longer has any power over us, but something still remains: the carnal (sinful) flesh. Our bodies have not been transformed yet:

For our citizenship is in heaven, from which we also eagerly wait for the Savior, the Lord Jesus Christ, who will transform our lowly body that it may be conformed to His glorious body, according to the working by which He is able even to subdue all things to Himself. (Philippians 3:20–21)

Our bodies will be transformed like Christ’s glorified body, and our carnal flesh will be no more (1 Corinthians 15:53). Basically, Christians still struggle with their flesh because God’s plan is not fully completed yet. Christ’s work on the Cross is complete, and now we look forward to the time when Jesus Christ will transform our bodies into something new and sinless (Romans 8:23). http://www.answersingenesis.org/articles/2010/11/30/struggle-with-the-flesh