In his book Newton: On the Christian Life, author Tony Reinke recalls a metaphor John Newton used to illustrate the effects of indwelling sin:

Imagine a Christian sitting down with a blank page and pen. He begins to write out his perfectly scripted life, explaining how he would love others, how he would structure his prayer life, or how he would [build a beautiful Christian family]. But indwelling sin and Satan crouch at his elbow, disrupting every pen stroke and messing up every word and sentence as our Christian friend tries to write the script.

At every point in the Christian life [our own flesh] and Satan jab our elbow, and our pen skids across the page as our perfect plan is reduced to scribbles. This is a metaphor of the Christian life with indwelling sin. Yet the biggest problem is that sin is not at our elbow—our sin is in us!