I talked about this briefly in another post. But let me talk about this again in regards to your question. There have been many thoughts about how the mind, body, essence of how a human being really works. For instance, Dallas Willard breaks down a person into "elements." Think about an onion with many layers, with the spirit being the innermost layer and the social relationships a person has as the outermost layer:
Elements of a Person: spirit --> mind --> body --> social
For Willard, the Spirit of a person is the inner core of being from which actions flow (this is what many would consider “the will”). Our spirit is the power to do what is good or evil. So the spirit is where sin first arises.
Think of it this way: You are confronted with a temptation to sin. Let's say your friends are talking badly about another friend behind her back (i.e. gossip). In order for you to commit sin, your spirit (your "will") must say “yes” to that sin. If there is no inner yes, there is no sin. Even if you think about sin, that’s not a sin (that’s not even temptation!). Remember, temptation is the thought plus the inclination to sin. It's when you linger over the thought. Sin is when we inwardly say yes to the temptation; when we decide to do it.
So summarily, NO - it is not a sin to think about stealing, lying and sexual thoughts. It is not even temptation. It is when you inwardly say yes to an action (and this includes thinking sinfully about something such as lusting over a person in your mind) and then decide to do it - that's when it becomes a sin. That is why the Bible tells us we must resist and run from temptation when we see it, rather than allowing our fascination to draw us closer. God provides for this in 1 Corinthians 10:13: "No temptation has seized you except what is common to man. And God is faithful; he will not let you be tempted beyond what you can bear. But when you are tempted, he will also provide a way out so that you can stand up under it."
[Answered by Pastor HM]
