15 Therefore the people of Israel is born, in such wise that it is made; nor do we take the assertion that it is born as contradictory to the fact that it is made. For it is a son by adoption, not by generation; nor is this its true character, but its title. For although the words, My firstborn are written of it; there is yet a great and wide difference between My beloved Son, and My firstborn son. For where there is birth, there we see, My beloved Son; but where there is a choice from among the nations, and adoption through an act of will, there is My firstborn son.
Here the people is God's, in regard to its character as firstborn; in the former case the fact that He is God's, relates to His character as a Son. Again, in a case of birth the father's ownership comes first, and then his love; in a case of adoption the primary fact is that the son is made a firstborn, and then comes the ownership. Thus to Israel, adopted for a son out of all the peoples of the earth, properly belonged the character of a firstborn; but to Him alone, Who is born God, properly belongs the character of a Son.
Accordingly there is no true and complete birth where sonship is imputed rather than real: since it is not doubtful that that people, which is born into a state of sonship, is also made. But since it would not have been what it is now become, and inasmuch as its birth is but a name for its being made, it has no true birth, since it was something else before it was born. And for this reason it was not before it was born, that is, before it was made, because that which is a son from among the nations was a nation before it was a son: and accordingly it is not truly a son, because it was not always a son.
But God Only-begotten was neither at any time not a Son, nor was He anything before He was a Son, nor is He Himself anything except a Son. And so He Who is always a Son, has rendered it impossible for us to think of Him that there was a time when He was not.
Source: On the Trinity (New Advent)