Abimelech learned that sin is sin against God

We underestimate sin. We underestimate its power. We underestimate its effect. We underestimate its presence. And we certainly underestimate how God feels about it. Most of all, we underestimate against whom we are really sinning against.

The story in Acts 9 is familiar. The scene is the road to Damascus, and a man named Saul was breathing out threats and killing the Lord's disciples. Jesus spoke to Saul, soon to be Paul, and asked in verse 4: "Saul, Saul, why are you persecuting Me?"

What? Saul was killing the disciples, not Jesus. Ah, but this verse shows us how intimately Jesus is involved in our lives, how tightly we are one, how, when you come against one of His children you come against Him. Saul was not just railing against the name of Christ in the disciples, but coming against Christ Himself.

for the union between Christ and his people is so close, that what is done to them is done to him. ~Gill's Exposition
There is another scene in the Bible that displays a similar sentiment. It's in Genesis 20. Abraham fears for his life and lies to Abimelech king of Gerar that Sarah is his sister so they won't kill Abraham in trying to get to Sarah. Based on what Abraham said, Abimelech took Sarah.

Now comes the interesting part. Genesis 20:3 says what happened next was,

God came to Abimelech in a dream by night and said to him, “Behold, you are a dead man because of the woman whom you have taken, for she is a man’s wife.”

I wrote about one aspect of the verse a few days ago, here in an essay titled Beware of desiring a dream/vision, word from the Lord. God said to Abimelech, 'Behold you are a dead man'?! The dreams and words from God the false teachers say they receive today are far from that powerful - and deadly. Anyway, Abimelech pleads his case. He replies in Genesis 20:4-5,

“Lord, will you kill an innocent people? Did he not himself say to me, ‘She is my sister’? And she herself said, ‘He is my brother.’ In the integrity of my heart and the innocence of my hands I have done this.”

God agrees with him. Abimelech had gone forward based on the information that was given to him, that Sarah was single. Given that he thought she was single, he took Sarah. However it was still sin.

Here is the climactic verse for the point of this essay:

God said to him in the dream, “Yes, I know that you have done this in the integrity of your heart, and it was I who kept you from sinning against me." (Genesis 20:6). emphasis added.

There it is again. Abimelech learned the hard way that he was sinning. Worse, he learned that was sinning against God Himself. It didn't matter that Abimelech hadn't known Sarah was married. It didn't even matter that Abimelech didn't know God. Note that God did not say, 'you would have been sinning against Abraham, the husband.' Abimelech would have been sinning against Abraham. Ultimately though, all sin is performed against our Holy God. The King would be sinning against God if he had gone through with what he'd intended with Sarah. Here comes to mind the axiom, 'Ignorance of the law is no excuse".

Genesis 39:9 recounts a similar scene,

Joseph was being tempted to commit adultery with Potiphar's wife. In resisting her, he said, "My master has withheld nothing from me except you, because you are his wife. How then could I do such a wicked thing and sin against God?" It is interesting that Joseph did not say that his sin would be against Potiphar. This isn’t to say that Potiphar would be unaffected. But Joseph's greater loyalty was to God and His laws. It was God he did not want to offend. ~GotQuestions

Psalm 51:4 says "Against you, you only, have I sinned and done what is evil in your sight, so that you may be justified in your words and blameless in your judgment."

All our sins are against a just and Holy God. Our sins might be toward a co-worker, a sibling, a passerby. But all sins are against God. We should keep this in mind.

No proof of the fullness of sin, after all, is so overwhelming and unanswerable as the cross and passion of our Lord Jesus Christ and the whole doctrine of His substitution and atonement. Terribly black must that guilt be for which nothing but the blood of the Son of God could make satisfaction. Heavy must that weight of human sin be which made Jesus groan and sweat drops of blood in agony at Gethsemane and cry at Golgotha, "My God, My God, why have You forsaken Me?" (Matt. 27:46). Nothing, I am convinced, will astonish us so much, when we awake in the resurrection day, as the view we will have of sin and the retrospect we will take of our own countless shortcomings and defects. Never until the hour when Christ comes the second time will we fully realize the "sinfulness of sin." JC Ryle

We should keep this in mind also-

but God shows his love for us in that while we were still sinners, Christ died for us. (Romans 5:8)

Greater love has no one than this, that he lay down his life for his friends. (John 15:13)

Our knowledge of the sinfulness of sin and our resulting feeling of guilt is tempered by our love for Jesus who atoned for that sin. It is this love for Him that makes us want to mortify it in ourselves all the more. And so it goes, until the Day when we awaken, and see the true effect of sin, and say "what hath God wrought!" (cf Numbers 23:23)