Charles Ellicott Commentary Genesis 26:28-29

"And they said, We saw plainly that Jehovah was with thee. And we said, Let there now be an oath betwixt us, even betwixt us and thee, and let us make a covenant with thee, that thou wilt do us no hurt, as we have not touched thee, and as we have done unto thee nothing but good, and have sent thee away in peace. Thou art now the blessed of Jehovah." — Genesis 26:28-29 (ASV)

Let there be now an oath. —The word literally signifies a curse. Each side uttered an imprecation, with the prayer that it might fall upon himself if he broke the terms of the covenant.

Let us make a covenant. —Hebrew, cut. (See Note on Genesis 15:10; Genesis 15:18; where you can also see the explanation of this use of the word curse.)

The Lord was with thee ... blessed of the Lord. —This use of the word “Lord,” that is, Jehovah, is very remarkable. In Genesis 21:22-23 Abimelech uses the term Elohim, God, in accordance with the careful discrimination in the use of the names of the Deity often previously referred to.

By the long residence, first of Abraham and then of Isaac, in their territory, the Philistines would indeed have become better acquainted with the religion of the patriarchs; but as Jehovah was not their special title for the Deity (Exodus 6:3), we must conclude, with Rosenmüller, that it was Moses who wrote Jehovah in place of the word actually employed by Abimelech. We gather, however, that the king did not use any generic or heathen names of the Deity, but that by which the patriarchs worshipped their covenant God, and his doing so was probably intended as an act of homage to Him.