11 Further, what terror had the pain of death for Him, to Whom death was an act of His own free will? In the human race death is brought on either by an attack upon the body of an external enemy, such as fever wound, accident or fall: or our bodily nature is overcome by age, and yields to death. But the Only-begotten God, Who had the power of laying down His life, and of taking it up again, after the drought of vinegar, having borne witness that His work of human suffering was finished, in order to accomplish in Himself the mystery of death, bowed His head and gave up His Spirit. If it has been granted to our mortal nature of its own will to breathe its last breath, and seek rest in death; if the buffeted soul may depart, without the breaking up of the body, and the spirit burst forth and flee away, without being as it were violated in its own home by the breaking and piercing and crushing of limbs; then fear of death might seize the Lord of life; if, that is, when He gave up the ghost and died, His death were not an exercise of His own free will.
But if He died of His own will, and through His own will gave back His Spirit, death had no terror; because it was in His own power.
Source: On the Trinity (New Advent)