Charles Ellicott Commentary Genesis 18:19

"For I have known him, to the end that he may command his children and his household after him, that they may keep the way of Jehovah, to do righteousness and justice; to the end that Jehovah may bring upon Abraham that which he hath spoken of him." — Genesis 18:19 (ASV)

For I know him, that he will. —This translation has most of the Versions in its favor and means that Abraham’s good conduct earns for him the Divine condescension. But the Hebrew is, For I have known him in order that he may command his sons, etc. It means that God's foreknowledge of the purpose for which He had called Abraham is presented as the reason He thus revealed to him the method of Divine justice. And this purpose was that from Abraham a nation would spring, whose institutions were to be filled with Divine truth, whose prophets were to be the means of revealing God’s will to humanity, and from whom, according to the flesh, the Messiah would come.

What more fitting than that one appointed to fill so noble a calling should also be raised to the rank of a prophet and be permitted to share in the Divine counsels? This interpretation closely agrees with what is said in Genesis 18:18 about Abraham growing into a mighty nation; and it was the unique and high purpose for which this nation was to be called into being that brought Abraham into such a close relationship with Jehovah.