How did Christ humble himself in his conception and birth?
Christ humbled himself in his conception and birth, in that, being from all eternity the Son of God, in the bosom of the Father, he was pleased in the fulness of time to become the son of man, made of a woman of low estate, and to be born of her; with divers circumstances of more than ordinary abasement.1
Scripture proofs
The Westminster Assembly's proof texts (KJV). The bracketed numbers in the answer above mark the clause each set of references supports; expand a row to read the verses.
1 John 1:14, 18; Gal. 4:4; Luke 2:7
John 1:14 And the Word was made flesh, and dwelt among us, (and we beheld his glory, the glory as of the only begotten of the Father,) full of grace and truth.
John 1:18 No man hath seen God at any time; the only begotten Son, which is in the bosom of the Father, he hath declared him.
But when the fulness of the time was come, God sent forth his Son, made of a woman, made under the law,
And she brought forth her firstborn son, and wrapped him in swaddling clothes, and laid him in a manger; because there was no room for them in the inn.
This doctrine across the Standards
Where the Confession and the Westminster Shorter Catechism treat the same matter, so the question can be studied alongside its parallel statements.
In the Westminster Confession
VIII.4 This office the Lord Jesus did most willingly undertake, which, that he might discharge, he was made under …
This office the Lord Jesus did most willingly undertake, which, that he might discharge, he was made under the law, and did perfectly fulfill it; endured most grievous torments immediately in his soul, and most painful sufferings in his body; was crucified, and died; was buried, and remained under the power of death, yet saw no corruption. On the third day he arose from the dead, with the same body in which he suffered; with which also he ascended into heaven, and there sitteth at the right hand of his Father, making intercession; and shall return to judge men and angels at the end of the world.
→ VIII.4 · Of Christ the MediatorIn the Westminster Shorter Catechism
Q.27 Wherein did Christ's humiliation consist?
Christ's humiliation consisted in his being born, and that in a low condition, made under the law, undergoing the miseries of this life, the wrath of God, and the cursed death of the cross; in being buried, and continuing under the power of death for a time.
→ Open Q.27Q.28 Wherein consisteth Christ's exaltation?
Christ's exaltation consisteth in his rising again from the dead on the third day, in ascending up into heaven, in sitting at the right hand of God the Father, and in coming to judge the world at the last day.
→ Open Q.28