Tafsiran Kejadian 17:4: Ramban, Kli Yakar, Sforno & Kitzur Ba'al HaTurim

BEHOLD, MY COVENANT IS WITH THEE, AND THOU SHALT BE THE FATHER OF A MULTITUDE OF NATIONS. This refers to the covenant of the circumcision, as He will explain, “This is the sign of the Covenant,⁴⁰¹In Verse 10: This is my covenant. and after the covenant you will be the father of a multitude of nations.”
Blessed be the Lord! By Him alone are actions weighed, for He preceded and commanded Abraham to enter into His covenant to be circumcised before Sarah became pregnant so that his seed would be holy.

“Behold, My covenant is with you.” The word “behold” [hineh] indicates something that already exists. One can explain that “My covenant is already present within you,” and this is indeed true, because this external circumcision serves a noble purpose — it provides proof of the circumcision of the heart and its purity. Without doubt, even before the physical circumcision, Abraham already had a pure and circumcised heart. Therefore, the Holy One, blessed be He, said to him, “I know that My covenant is already with you because you are pure of heart. However, since I now want to make you a father of many nations, I need to provide an explanation to all the people of the world as to why I am appointing you as the father of many nations. This is to publicize your virtue and the purity of your heart before all the nations, through this external circumcision which testifies to the purity of the heart, so that your generations will know that I justly made you a father of many nations.” This is what is meant by And I will establish My covenant between Me and you — meaning, that same internal covenant of the heart which was previously hidden between Me and you, and which all the people of the world did not know about, I am now raising as a banner of that covenant so that all the people of the world will know of it. This is what is meant by for an everlasting [leolam] covenant — meaning, that covenant which was between Me and you will now be a covenant for the entire world, as all the people of the world [haolam] will know of it and reflect upon it.

אני הנה בריתי אתך, "I am now doing what I had told you, to make the covenant actual."