7 The believer in the true doctrine of the gospel will understand that Christ is not reproached by Moses when he speaks of Him as cursed, not in His divine majesty, but as hanging on the tree as our substitute, bearing our punishment, any more than He is praised by the Manichæans when they deny that He had a mortal body, so as to suffer real death. In the curse of the prophet there is praise of Christ's humility, while in the pretended regard of the heretics there is a charge of falsehood.
If, then, you deny that Christ was cursed, you must deny that He died; and then you have to meet, not Moses, but the apostles. Confess that He died, and you may also confess that He, without taking our sin, took its punishment. Now the punishment of sin cannot be blessed, or else it would be a thing to be desired. The curse is pronounced by divine justice, and it will be well for us if we are redeemed from it. Confess then that Christ died, and you may confess that He bore the curse for us; and that when Moses said, "Cursed is every one that hangs on a tree," he said in fact, To hang on a tree is to be mortal, or actually to die.
He might have said, "Cursed is every one that is mortal," or "Cursed is every one dying;" but the prophet knew that Christ would suffer on the cross, and that heretics would say that He hung on the tree only in appearance, without really dying. So he exclaims, Cursed; meaning that He really died. He knew that the death of sinful man, which Christ though sinless bore, came from that curse, "If you touch it, you shall surely die." Thus also, the serpent hung on the pole was intended to show that Christ did not feign death, but that the real death into which the serpent by his fatal counsel cast mankind was hung on the cross of Christ's passion.
The Manichæans turn away from the view of this real death, and so they are not healed of the poison of the serpent, as we read that in the wilderness as many as looked were healed.
Source: Reply to Faustus the Manichaean (New Advent)