I think the greatest mystery in all of the universe, and by “mystery” I mean that which we as creatures cannot wrap our finite minds around so as to comprehend, is, not the trinity, and not that he made the universe ex nihilo (out of nothing), and not the virgin birth, but the greatest mystery is his divine love for us. The love of the Father predestining us, the love of the Son dying for us, and the love of the Spirit giving us a new birth.
This Trinitarian divine love that would reach down and save us unworthy sinners is the greatest confounding mystery in all of the Bible. And this mystery will only be more palpable on that day when we meet our Savior face to face.