Question 14: The term Holy Trinity causes also the following problem: How would it have been possible for one of the three divine persons to leave [the Holy Trinity], to enter Mary’s womb, to mix with this mortal world and to assume the human form? For, if God were a trinity then it would not be possible for only one of the three of the trinity to come down to earth separately.
Answer: Theme 2 (The divinity of Jesus) and theme 5 (God, the three-in-one) have shown that in Jesus, the Messiah, the uncreated son of God became man. Read: Heb 1; Eph 1; Col 1:12-20; Phil 2:5-11.
Read also theme 3 (Cross , sin and redemption, III, 2 and 2.1).