ETERNITY IS NOT ENDLESS TIME or the opposite of time. It is the essence of time.
If you spin a pinwheel fast enough, all its colors blend into a single color—white—which is the essence of all the colors of the spectrum combined.
If you spin time fast enough, time-past, time-present, and time-to-come all blend into a single timelessness or eternity, which is the essence of all times combined.
As human beings we know time as a passing of unrepeatable events in the course of which everything passes away—including ourselves. As human beings, we also know occasions when we stand outside the passing of events and glimpse their meaning. Sometimes an event occurs in our lives (a birth, a death, a marriage —some event of unusual beauty, pain, joy) through which we catch a glimpse of what our lives are all about and maybe even what life itself is all about, and this glimpse of what "it's all about" involves not just the present, but the past and future too.
Inhabitants of time that we are, we stand on such occasions with one foot in eternity. God, as Isaiah says (57:15), "inhabiteth eternity," but stands with one foot in time. The part of time where he stands most particularly is Christ, and thus in Christ we catch a glimpse of what eternity is all about, what God is all about, and what we ourselves are all about too.