Genesis 22:16

and said, By My Word have I sworn, saith the Lord, forasmuch as thou hast done this thing, and hast not withheld thy son, thy only begotten,

BECAUSE THOU HAST DONE THIS THING. In the beginning He promised him that he would increase his descendants as the stars of heaven and the dust of the earth, but now He gave him the additional assurance that because thou hast done this great deed, He swore by His Great Name [that He would increase his descendants as the stars of heaven, and as the sand which is upon the seashore], and that his seed will possess the gate of its enemies. Thus Abraham was assured that no sin whatever would cause the destruction of his descendants, nor would they fall into the hand of their enemies and not rise again. Thus this constitutes a perfect Divine assurance of the redemption which is destined to come to us.

נאם ה' כי יען אשר עשית, “I, G’d, say that because you have done this I will bless you in an increased measure.”

ויאמר בי נשבעתי, G’d added an oath to the blessing He had already bestowed on Avraham. When G’d swears an “oath” it is as if He were to say “by My life, etc.” Just as He is eternal and therefore will be able to keep His oath, so He can keep His oath to the children and grandchildren of the ones to whom He promises something by oath. While it is true that G’d does not need to swear an oath to assure us that He will keep His word, the new element in G’d swearing an oath is that even if the recipient of the oath did not keep his part of the bargain, i.e. sinned in the meantime, G’d promises that this will not invalidate His blessing, the one He confirms by an oath. Furthermore, He added another dimension to the existing blessing,