God became the Son by a metaphysical uniting of human nature to His one person through the miraculous virgin conception. God became something in time that He was not previously in eternity. There was no change in God's essential being (for He remained the same), but there was a change in God's manner of existence. As the church fathers taught, 'He became what He was not while remaining what He was.' Such a teaching stresses God's immutability, and yet acknowledges God's new manner of existence as a genuine human being (in addition to His continued existence beyond the incarnation).
What looks difficult to you, Blouen? 'What he was not' would probably look more grammatically correct put as 'What He had not been' previously... However, the notion of time and changeability doesn't work in relation to God, hence, it seems, the use of 'was' here...Paradoxical, of course, but then the whole concept of Incarnation is paradoxical.
Hope that helps.
Are there any theologians around here?
I got it now, I read the whole article and understood what the author means about it.
The author here is "Oneness" in theology, saying that God manifested himself in the flesh as man in Jesus Christ. So when God incarnated, He became what He was not, in the essence of humanity, while remaining what He was in the essence of being God.