ORLANDO, FLORIDA—A few days ago, I embarked on the annual tradition of setting up our family Christmas tree. As I wrote last year, the agony of pine needles and tangled lights is outweighed by the joy of seeing my grandchildren, Jo and Liam, caught up in the wonder and expectation of the Advent season.

Part of the reason why Jo loves December so much is that her birthday is just ten days before Christmas. Since December 15 and December 25 are so close, we all work hard to distinguish between Jo’s birthday and Christmas, which we have explained to her is Jesus’ birthday.

When looking at an Advent calendar in the kitchen, Jo pointed to December 15 and said, “This is my birthday, right?”

“Yes!” My son William, her dad, replied. “You are going to be four years old on your birthday.”

Pointing to December 25, Jo said, “And this is Jesus’ birthday. How old is he going to be—two or four?”

William smiled awkwardly, having no idea how to answer the question. “Well, Jesus had a second birthday and then two years later, he had a fourth birthday, just like you will be having.”

For the record, I don’t know if Jews in first-century Palestine celebrated birthdays the way we do. But Jo’s question reminds us of the mystery of the incarnation—that Jesus, like Jo, had a fourth birthday.

Jesus, like Jo, was a three-year-old bursting with energy who occasionally made big a mess in Joseph’s carpentry shop.

Jesus, like Jo, was a three-year-old who made his parents laugh with his three-year-old sayings and pronunciations. (Josephine calls the month of December, “Becender.”)

Jesus, like Jo, was a three-year-old who needed mommy when he skinned his knee.

Jesus, like Jo, was a three-year-old who, on his best days, made Mary and Joseph wish that he would never grow up (and on his worst days, made Mary and Joseph wish he would hurry up and become an adult).

The mystery of the incarnation is that this same three-year-old—who could act like my granddaughter Josephine—was at the same time “begotten from the Father before all ages, God from God, Light from Light, true God from true God, begotten, not made; of the same essence as the Father,” as it says in the Nicene Creed.

The mystery of the incarnation is that this three-year-old, who occasionally could make a real mess in his earthly father’s workshop, was present with His Heavenly Father at the creation of the universe.

The mystery of the incarnation is that this three-year-old, who was still learning to speak and form words in his mother tongue (Aramaic), was the eternal Word of God—through whom all things (including our tongues) were made.

The mystery of the incarnation is that this three-year-old, who needed mom to wipe the blood off his knees and the tears from his eyes, was the Lamb of God—who was destined to weep and bleed and bear sins of the world.

The mystery of the incarnation is that this three-year-old, who was the beloved son of Mary and Joseph, was at the same time the beloved Son of God.