Leviticus 26:45 is designed to recall the events of the Mosaic covenant which the Lord had established with the nation at Mount Sinai. This event would still have been fresh in the minds of the original hearers. For subsequent generations, especially those who may be in exile, this event would be in the past.
45 But I will for their sake remember the covenant with their forefathers, whom I brought out of the land of Egypt in the sight of the nations, that I might be their God: I am the LORD.”