What is the battle between the flesh and the Spirit in a believer?
The battle is the conflict between old Adam, the flesh, and the indwelling Spirit. The flesh lusts and wars against the Spirit, while the Spirit wars against old Adam. Believers are to walk in the Spirit—their position in Christ—so they will not give in to old desires. The side fed best wins.
Les teaches that verse 17 proves the point: the flesh, old Adam, lusts or wars against the Spirit, and the Spirit—the indwelling Spirit that is now part of the believer—wars against old Adam. Therefore, believers are to walk in the Spirit, that position of being in Christ, and then they will not give in to the old desires of old Adam. Les also repeats an anecdote about a man who had lived in degradation and alcoholism, was saved miraculously, but had to pass his old bar every day to get his mail. He described the struggle as two little dogs inside having a rip-roaring fight: a black dog saying go back to the old lifestyle, and a white dog saying go get the mail. Asked which dog wins, he said, “the one I feed the best.”