What does Genesis 20:6 mean?

God has appeared to king Abimelech in a nighttime dream with a startling message: You are going to die from your current illness, because you took a married woman as your wife. Abimelech's response has been that not only had he not yet slept with Sarah, he had acted with integrity. He believed Abraham's lie that Sarah was merely his sister. Abimelech's question to the Lord from verse 4 still hangs in the air: Will you kill the innocent?

Now the Lord relieves the tension a little. He agrees that Abimelech has acted with integrity. He also reveals that He, the Lord, has kept Abimelech from even touching Sarah. This is part of God's faithfulness in protecting His promise to Abraham and Sarah, that they would soon have a natural-born son (Genesis 17:15–16). Making it clear that Sarah and Abimelech were not intimate is key to this passage.

Though the text doesn't say so here, verse 17 reveals that Abimelech had been ill. In Genesis 12, when Abraham lied in the same way and an Egyptian Pharaoh had taken Sarah as his wife, God had sent plagues on the Pharaoh's household (Genesis 12:17). Abimelech's illness may even have been why he has been unable to touch Sarah and complete his sin against the Lord.

Notice that God describes this potential adultery as a sin against Himself and not just against Sarah and Abraham. Later, God will make abundantly clear to His people Israel that adultery is strictly forbidden in all cases.