In James (1:14) we read that “But every man is tempted, when he is drawn away by his own lust, and enticed.” which implies that we are ostensibly our own worst enemy. Meaning that the reason we sin is solely because of our own sinful nature and not because of some outside influence (the devil). However, in 1 Peter (5:8) we read “your adversary the devil, as a roaring lion, walketh about, seeking whom he may devour” and again in 1 John (4:4) we read “greater is he that is in you, than he that is in the world.” These two scriptures would imply that temptation by the devil does exist and that (at least some of) our sin IS due to the devil.
James was right when he said we are drawn away by our own lust and enticed. This is why Peter cautioned us to be self-controlled and alert in 1 Peter 5:8. The devil is looking for someone whose lack of control makes him or her a target. The word translated enticed in James 1:14 literally means to be caught by baiting. Satan places his bait everywhere, but if we’re self-controlled we won’t be tempted by it.
In 1 John 4:4 the issue is false teaching. Because we’ve asked Jesus to come into our heart we have the discernment to recognize the false teaching that comes from the spirit of anti-Christ and overcome it.
In summary, sin always begins in the heart of the sinner. Satan’s bait is a constant in our environment. The variable is whether we allow ourselves to be drawn away and enticed by it. If we’re not guarding against it, our sin nature will always take the bait.