8 Think not therefore that this generation is human, nor as Abraham begot Isaac. For in begetting Isaac, Abraham begot not what he would, but what another granted. But in God the Father's begetting there is neither ignorance nor intermediate deliberation. For to say that He knew not what He was begetting is the greatest impiety; and it is no less impious to say, that after deliberation in time He then became a Father. For God was not previously without a Son, and afterwards in time became a Father; but has the Son eternally, having begotten Him not as men beget men, but as Himself only knows, who begot Him before all ages Very God.
Source: Catechetical Lectures (New Advent)