Packer goes on to explain that the greatest mystery is not the Good Friday story, nor the Resurrection story, but it is, in fact, the Christmas story. He says, "Here are two mysteries for the price of one--the union of Godhead and manhood in the person of Jesus."
He goes on to say the reason many have difficulties with the virgin birth, the miracles, the atonement, and the resurrection start from a misbelief about the Incarnation.
Perhaps rhetorically, he ponders an even better question:
"It is not strange that he (the Son of God), the Author of Life(italics mine), should rise from the dead. If he was truly God the Son, it is much more startling that he should die than that he should rise again.
'Tis mystery all! The Immortal dies,' wrote Wesley; but there is no comparable mystery in the Immortal's resurrection. And if the immortal Son of God did submit to taste death, it is not strange that a death should have saving significance for a doomed race. Once we grant that Jesus was divine, it becomes unreasonable to find difficulty in any of this....The Incarnation is in itself an unfathomable mystery, but it makes sense of everything else that the New Testament contains."
From Knowing God, chapter 5, pgs 52-54. by J.I. Packer