Genesis 21:22

And it was at that time that Abimelek and Phikol, chief of his host, spake to Abraham, saying, The Word of the Lord is in thine aid in all whatsoever thou doest.

ויהי בעת ההיא, at the time when Yitzchok was born by Sarah and Avimelech had become a first-hand witness to the miracles G’d had performed for Avraham. This is why he and his chief general were now interested in concluding an alliance with Avraham. Every time the expression בעת ההיא occurs it must be understood in terms of happenings immediately preceding the ones to which the next paragraph is linked.

אלוקים עמך, seeing that G’d clearly is on your side I am afraid of you. I am not afraid of your personal strength, but of that of your G’d. This is why I ask you to swear to me. [so that you cannot appeal to G’d’s assistance in any grievance you would have against me. Ed.]

ויהי בעת ההיא, at the time Yitzchok was born, or at the time he was weaned. At any rate, Avraham was still in the land of the Philistines. This is why the Torah now reports Avimelech saying that he was interested in receiving assurances from Avraham. Had Avraham already left the land of the Philistines prior to this, the Torah would have had to report that Avimelech traveled to Avraham in order to suggest the treaty he wanted to conclude. When he had seen that a son had been born to Avraham, in his old age, and they saw that Sarah was the one who had given birth to him, they began to take seriously the promises made by G’d to Avraham concerning the future of his descendants. The reason that Phicol the head of Avimelech’s army was mentioned, was to underline that the words Avimelech addressed to Avraham had been what his army chief had proposed.

אלוקים עמך, “G–d is on your side.” Originally, they had thought that if Avraham was such a righteous individual he would not have expelled his firstborn son. After observing Avraham’s lifestyle, his success, and good deeds, they had come to the conclusion that his G–d had approved all of what he had done.