It's a very old question ... probably as old as mankind itself, because with one notable Exception, all men and women are sinful creatures by nature. It's primarily a matter of concern for Christians, who have received Jesus Christ according to John 1:12: because, let's face it, "everyday" sin is not a big issue in the minds of most unbelievers, if they acknowledge the reality of sin at all. We live in a world in which psychologists and philosophers have been telling us, for at least a century or two, that there's no such thing as "sin" at all; that everything is relative. But people who think clearly, and independently, usually know better. You might not like the word "sin" (who does?), preferring "mistake" or "shortcoming," but there's a difference .... and most people understand that, even if they don't like to think about it.
Fortunately, the question "When does a sin become a sin" has an answer. After all, Christian thinkers have been studying this sort of thing for a long, long time. In Christian theology, the study of sin is called "hamartiology," from the Greek ἁμαρτία, meaning "sin." The subject of temptation is called peccability. Those are words that you're not likely to hear in everyday conversation, but the study of these things, as human phenomena, can be very helpful on a practical level.
We've discussed formal theology very little in this blog, for two reasons: In the first place, "theology" is frequently a substitute for the study of God's word itself, which is the source of all truth. In the second place, theologians, from Augustine to Bonhoeffer, are human beings, with human limitations, and they very often get it wrong. However, they've occasionally made some positive contributions, and the study of sin is one of them.
And, as we said, it's quite practical. This little snippet of theology has been very helpful to me, in my own life, and it might be helpful to you, as well. The classic theologians broke down what we've called "the pathology of temptation" into four stages .... and anybody who's aware of them can observe them, and recognize them, very easily.
The first stage in temptation is called presentation. This is when you're confronted with an object or a circumstance that could become sinful. It could be a bottle of whiskey, an "off-limits" member of the opposite sex, a sudden financial windfall that you could hide on your taxes, or a thousand other things. You see it: you're presented with it. But merely being presented with it doesn't involve sin. If it did, we'd all be sinning 24/7.
The second stage is called illumination. This is when you understand what you're seeing, and the possibilities at hand .... and the moral aspects involved. A woman in an office sees a co-worker smiling at her in a certain way (that's the presentation), and she thinks, "Hmmm ... you know, he's really interesting. Not like my husband, at all! And from the look on his face, I'll bet we could get together, and nobody would be the wiser." Or the man in the store sees the open cash drawer, and thinks, "I'll bet they wouldn't miss a few twenties or so .... and I probably wouldn't get caught...." Or you're looking for something on Google, and suddenly you're at one of "those" websites. It's right there in front of you, and nobody would know if you spent some time there. That's illumination: realizing the possibilities involved in the object or situation. But even this isn't sinful; it's just the way our minds work.
The third stage is called debate. This is where it gets tricky, and more than tricky: this is when you actually consider getting involved with the object or situation that's potentially sinful. You're no longer saying that you could "drown your sorrows" with the booze, or steal the money, or check out the pornography; at this stage, you're actually trying to decide whether or not to do it. "Is it worth the risk? It wouldn't really hurt anybody, would it? And surely God understands, doesn't He? After all, it's not like I'm killing anybody ... and if I don't get caught ... But no, it's wrong. I shouldn't do it. I know better. And yet....." That's debate: you're trying to decide whether or not to do the thing.
The fourth and final stage is decision. That's when you give in to the temptation, and say, "Damn the consequences, I'm going to do it! It's my life, and I have a right to do what I want!" And you do it, whatever "it" is. The Holy Spirit tells you "No," and/or your conscience tells you "No," but you do it anyway.
But decision is not where "sin becomes sin." That happened during the third stage, debate, when you were actually trying to decide whether or not to do it. Up until that time, you hadn't sinned; you'd merely been tempted. And temptation, obviously, isn't sin: even Jesus was tempted, by Satan in the desert. But He didn't debate about it, or say, "Maybe Satan's got a point!" Debate is where sin enters in.
I say it very carefully, and very reverently: God isn't so much interested in what we do .... as what we want to do. That's why Jesus said, whosoever looketh on a woman to lust after her hath committed adultery with her already in his heart (Matthew 5:28). That's why John, writing by inspiration of the Holy Spirit, said, Whosoever hateth his brother is a murderer: and ye know that no murderer hath eternal life abiding in him (1 John 3:15). Regardless of whether we ever reach the point of decision, and commit the actual act, the sin begins with the debate: for man looketh on the outward appearance, but the LORD looketh on the heart (1 Sam. 16:7). And the Lord - - - not your pastor, your Pope, or your mother - - - is the One Who decides what sin is.
But it doesn't have to get as far as the actual act, or even debate. If a man or woman has been born again, according to John 3:3-7, he or she has been given exceeding great and precious promises: that by these ye might be partakers of the divine nature, having escaped the corruption that is in the world through lust (2 Peter 1:4). Chief among these, in this matter of sin and temptation, is 1 Corinthians 10:13: There hath no temptation taken you but such as is common to man: but God is faithful, who will not suffer you to be tempted above that ye are able; but will with the temptation also make a way to escape, that ye may be able to bear it.
Never mind the theological terms; just remember the four stages in the pathology of temptation. Presentation, simply seeing an object of temptation, isn't a sin. Neither is illumination, when you realize what's available. Sin begins with debate: when you start figuring the odds, wondering whether you can or should do it. That's what leads to the actual decision, and the act of sin.
But even if we blow it, even if we ignore the warnings and walk headlong into sin, forgiveness is available. Because we're not alone. For we have not an high priest which cannot be touched with the feeling of our infirmities; but was in all points tempted like as we are, yet without sin (Hebrews 4:15). Although He never sinned, our God was once a Man, walking in the world, and He knew temptation. He understands. And, although we will always have to reap what we sow (Galatians 6:7), we can acquire forgiveness as soon as we sincerely ask for it: 1 John 1:9.
But how much better to avoid it in the first place!