It was not the triune God but the second person of the Trinity that assumed human nature. For that reason it is better to say that the Word became flesh than that God became man. At the same time we should remember that each of the divine persons was active in the incarnation, Matt. 1:20; Luke 1:35; John 1:14; Acts 2:30; Rom. 8:3; Gal. 4:4; Phil 2:7. This also means that the incarnation was not something that merely happened to the Logos, but was an active accomplishment on His part. In speaking of the incarnation in distinction from the birth of the Logos, His active participation in this historical fact is stressed, and His pre-existence is assumed. It is not possible to speak of the incarnation of one who had no previous existence.
L. Berkof, Systematic Theology, 4th ed. (Grand Rapids: Eerdmans, 1941), 333.