Genesis 20:5-7 meaning

Abimelech reminds God of what Abraham and Sarah had told him. "Did he not himself say to me, 'She is my sister'?" Literally, the expression, in the integrity of my heart means "with a perfect heart" or integrity in his conduct. So far, Abimelech in defending himself has appealed to God's sense of justice and brought attention to the testimony of both Abraham and Sarah; now he states in both thoughts and actions he had no evil intentions, only good faith. 

Abimelech was acting without knowing he was doing wrong. In other words, he had innocence of his hands, his behavior was free from guilt (Psalm 24:4, 26:6, 73:13; Hosea 8:5; Amos 4:6). 

God now counters the defense of the king saying, "Yes, I know…and I also kept you from sinning against Me." In response to the king's first defense of innocence, God replies that He knew Abimelech did not know Sarah was married. But God adds that it was He who kept the king from sinning. God did not let Abimelech touch her, (approach Sarah sexually). Sometimes God intervenes in the affairs of humans (1 Samuel 25:39; Genesis 12:17, 26:11). God prevented anything that might discredit the birth of Sarah's promised child (Genesis 21; Matthew 1:25). 

This episode largely mirrors Abraham and Sarah's experience in Egypt, when Pharaoh took Sarah into his harem. God's approach in this episode with Abimelech makes it likely that God also prevented Sarah from being sexually defiled by Pharaoh, although we are not directly told so in that story (Genesis 12:15-20). It is also interesting that Abraham repeated the same approach as in Genesis 12 in dealing with a concern that he might be killed by a local king who desired to take Sarah. It had worked before. God had protected them before, and did not tell Abraham not to use this ploy. So apparently they repeated what had worked before, and it worked again. We will see that Abraham and Sarah's son Isaac will also use this same tactic (Genesis 26:6-11). 

As for the king's second response to innocence, his integrity is put to the test. Now that he knows the truth, Abimelech must immediately return Sarah to Abraham and restore the man's wife. God commands Abimelech to return Sarah to her lawful husband, Abraham. God points out that Abraham is a prophet. Interestingly, this is another first in Genesis, the first use of the word prophet.

One function of a prophet is to intercede to others to God (1 Samuel 7:5-9, 12:19; Deuteronomy 9:20; Numbers 12:13; Amos 7:2, 5; Jeremiah 7:16, 15:1; Psalm 105:12-15). We have seen Abraham fulfill this role (Genesis 18:22-32). The other primary role of a prophet was to represent God to people, speaking and judging on His behalf.  Abraham was not a prophet in this sense, so far as we are told, as was Samuel, Elijah, Isaiah, or Jeremiah. They spoke for God through divine inspiration in order to guide the people of Israel, including predictions of the future. That God tells us Abraham is a prophet might indicate that Abraham spoke on God's behalf. 

It could be that Abraham spoke on God's behalf in a manner described in the New Testament, which encourages believers to seek the gift of prophecy:

But one who prophesies speaks to men for edification and exhortation and consolation (1 Cor 14:3).

It could also be that Abraham lived a life that spoke on behalf of God, in word and deed, and that fulfilled this sort of prophetic function. If so, that would indicate that each believer who is a child of Abraham by faith (Gal 3:7) would have the same opportunity.

It is also interesting to reflect on God's interaction with Abimelech. Abimelech readily acknowledges and humbles himself before God. And God speaks to Abimelech in the dream. We are not told if this is a one time occurrence or part of an ongoing relationship. But as we saw with Melchizedek, God is working on multiple fronts and engaging with many people. He is just recording this particular story for our instruction (1 Corinthians 10:1-11).