VI. The import of the Father's voice from the cloud
And so while He was yet speaking, behold a bright cloud overshadowed them, and behold a voice out of the cloud, saying, “This is My beloved Son, in whom I am well pleased; hear Him.” The Father was indeed present in the Son, and in the Lord's brightness, which He had tempered to the disciples' sight, the Father's Essence was not separated from the Only-begotten: but, in order to emphasize the two-fold personality, as the effulgence of the Son's body displayed the Son to their sight, so the Father's voice from out the cloud announced the Father to their hearing.
And when this voice was heard, “the disciples fell upon their faces, and were sore afraid,” trembling at the majesty, not only of the Father, but also of the Son: for they now had a deeper insight into the undivided Deity of Both: and in their fear they did not separate the One from the Other, because they doubted not in their faith. That was a wide and manifold testimony, therefore, and contained a fuller meaning than struck the ear. For when the Father said, “This is My beloved Son, in Whom, etc.,” was it not clearly meant, “This is My Son,” Whose it is to be eternally from Me and with Me?
Because the Begetter is not anterior to the Begotten, nor the Begotten posterior to the Begetter. “This is My Son,” Who is separated from Me, neither by Godhead, nor by power, nor by eternity. “This is My Son,” not adopted, but true-born, not created from another source, but begotten of Me: nor yet made like Me from another nature, but born equal to Me of My nature. “This is My Son,” “through Whom all things were made, and without Whom was nothing made” because all things that I do He does in like manner: and whatever I perform, He performs with Me inseparably and without difference: for the Son is in the Father and the Father in the Son, and Our Unity is never divided: and though I am One Who begot, and He the Other Whom I begot, yet is it wrong for you to think anything of Him which is not possible of Me.
“This is My Son,” Who sought not by grasping, and seized not in greediness, that equality with Me which He has, but remaining in the form of My glory, that He might carry out Our common plan for the restoration of mankind, He lowered the unchangeable Godhead even to the form of a slave.
Source: Sermons (New Advent)