8 What means, “With You is the beginning”? Suppose anything you please as the beginning. Of Christ Himself, it would rather have been said, You are the Beginning, than, With You is the beginning. For He answered to those who asked Him, “Who are You?” and said, “Even the same that I said unto you, the Beginning;” since His Father also is the Beginning, of whom is the only-begotten Son, in which Beginning was the Word, for the Word was with God. What then, if both the Father and the Son are the beginning, are there two beginnings?
God forbid! For as the Father is God, and the Son is God, but the Father and the Son are not two Gods, but one God: so is the Father Beginning and the Son Beginning, but the Father and the Son are not two, but one Beginning. “With You is the beginning.” Then it shall appear in what sense the beginning is with You. Not that the beginning is not with You here also. For have You not also said, “Behold, you shall be scattered, every man to his own, and shall leave Me alone; but I am not alone, because the Father is with Me”? Here therefore also, the beginning is with You.
For You have said elsewhere also, “But the Father that dwells in Me, He does His works.” “With You is the beginning:” nor was the Father ever separated from You. But when the Beginning shall appear to be with You, then shall it be manifest unto all who are made like You; since they shall see You as You are; for Philip saw You here, and sought the Father. Then therefore shall be seen what now is believed: then shall “the beginning be with You” in the sight of the righteous, in the sight of saints; the ungodly being removed, that they may not see the brightness of the Lord....
Source: The Enarrations, or Expositions, on the Psalms (New Advent)