There is a double nature in all believers

There is a double nature in all believers. Converted, renewed, sanctified as they are, they still carry about with them a mass of indwelling corruption, a body of sin. Paul speaks of this when he says, "I find a law, that, when I would do good, evil is present with me. For I delight in the law of God after the inward man. But I see another law in my members, warring against the law of my mind" (Romans 7:21-23). The experience of all true Christians in every age confirms this. They find within, two contrary principles, and a continual strife between the two. To these two principles our Lord alludes when He addresses His half-awakened disciples. He calls the one flesh and the other spirit. He says, "the spirit is willing, but the flesh is weak."

~ J.C. Ryle