7 For our sake, therefore, Jesus Christ, retaining all these attributes, and being born man in our body, spoke after the fashion of our nature without concealing that divinity belonged to His own nature. In His birth, His passion, and His death, He passed through all the circumstances of our nature, but He bore them all by the power of His own. He was Himself the cause of His birth, He willed to suffer what He could not suffer, He died though He lives for ever. Yet God did all this not merely through man, for He was born of Himself, He suffered of His own free will, and died of Himself.
He did it also as man, for He was really born, suffered and died. These were the mysteries of the secret counsels of heaven, determined before the world was made. The Only-begotten God was to become man of His own will, and man was to abide eternally in God. God was to suffer of His own will, that the malice of the devil, working in the weakness of human infirmity, might not confirm the law of sin in us, since God had assumed our weakness. God was to die of His own will, that no power, after that the immortal God had constrained Himself within the law of death, might raise up its head against Him, or put forth the natural strength which He had created in it.
Thus God was born to take us into Himself, suffered to justify us, and died to avenge us; for our manhood abides for ever in Him, the weakness of our infirmity is united with His strength, and the spiritual powers of iniquity and wickedness are subdued in the triumph of our flesh, since God died through the flesh.
Source: On the Trinity (New Advent)