3 He therefore, the Unbegotten, before time was begot a Son from Himself; not from any pre-existent matter, for all things are through the Son; not from nothing, for the Son is from the Father's self; not by way of childbirth, for in God there is neither change nor void; not as a piece of Himself cut or torn off or stretched out, for God is passionless and bodiless, and only a possible and embodied being could so be treated, and, as the Apostle says, in Christ dwells all the fullness of the Godhead bodily. Incomprehensibly, ineffably, before time or worlds, He begot the Only-begotten from His own unbegotten substance, bestowing through love and power His whole Divinity upon that Birth.
Thus He is the Only-begotten, perfect, eternal Son of the unbegotten, perfect, eternal Father. But those properties which He has in consequence of the Body which He took, are the fruit of His goodwill toward our salvation. For He, being invisible and bodiless and incomprehensible, as the Son of God, took upon Him such a measure of matter and of lowliness as was needed to bring Him within the range of our understanding, and perception, and contemplation. It was a condescension to our feebleness rather than a surrender of His own proper attributes.
Source: On the Trinity (New Advent)