Galatians 5:25
John 3:5-6 – Jesus answered, Verily, verily, I say unto thee, Except a man be born of water and of the Spirit, he cannot enter into the kingdom of God. That which is born of the flesh is flesh; and that which is born of the Spirit is spirit.
So here, we see what the opposite of walking by the Spirit is, namely, giving into the desires of the flesh. Remember, ‘flesh’ is an old, ordinary human nature that does not relish the things of God but prefers to get satisfaction from independence, power, prestige, and worldly pleasures. When we ‘walk in the Spirit’ we are not controlled by these drives.