ויהי אחר הדברים האלה. It was after these events. The events referred to are Abraham's various adventures, the tortuous route to becoming Isaac's father at an advanced age. In the meantime Isaac had grown up since we find Abraham described as "living in the land of the Philistines for many years," and Isaac was born during the first year of Abraham's stay there. The test assumed much greater impact in view of the repeated promises G'd had made to Abraham concerning Isaac and his future. To be told at this stage to offer Isaac as a sacrifice and not to hesitate or challenge G'd's command or even ask for an explanation, was a tremendous act of obedience. Isaac too deserves great credit for submitting. Another reason that the Torah wrote "after these events" is, that only now could G'd refer to Isaac as Abraham's only son. As long as Ishmael had not yet been expelled by Abraham the description "your only son" would not have been appropriate.