It isn’t really fair to change the definition of a word when you are responding to something I said

I didn’t think I was changing a definition, I thought I was clarifying one. Apologies.

…Perfection…means “unable to improve further”.

I agree with this definition.

It was only after the resurrection that he was unable to sin in his humanity.

But on City of God you said:

Instead, I believe that creatures with free will necessarily have the possibility of sinning.

1) As you said, free will means the possibility of sinning necessarily exists. I think you would agree that Christ has free will (see point 3). Therefore, the possibility must still exist that Christ could sin. If, Christ having free will, the possibility of sinning still exists, then He is essentially the same as He was prior to the resurrection regarding His perfection.

As far as I can tell, the reason He was sinless on Earth and will forever remain sinless is because of the presence of His Divinity, giving fidelity to His humanity. If He was human alone, as was the case with Adam #1, He would have in all likelihood fallen. It was Christ’s Divinity that maintained His sinlessness, in which case, His humanity has seen no effective improvement (as sin potential still must exist in no different a way than it did prior to the Crucifixion).

If He’s seen no effective change then He is as He has always been. Thus, if He is perfect now, He has always been perfect.

2) God resides outside of time. Therefore, true perfection is not momentary and must extend throughout time infinitely. If Christ was perfected upon the Resurrection, then this perfection would have necessarily been made manifest before the Crucifixion in order to truly be perfection (because post resurrection the “past-Christ” would have been imperfect thus keeping “present-Christ” imperfect). Thus, Christ would have been perfect before the Crucifixion.

3) Even on Earth Christ said that He has no will except that of the Father, which would seem to preclude the possibility of Christ sinning even prior to the Crucifixion, in which case, He would still have seen no improvement post-resurrection.

Goodness means without any flaw, lacking nothing as regards oneself. Christ in his humanity was always good. He was not always perfect. It was only after the resurrection that he was unable to sin in his humanity. That is an improvement. Therefore, he was not perfect in his humanity before. QED.

The “best we can do” is find a lamb without blemish. That is the closest to perfect that is conceivably possible — but that’s fine: this is simply foreshadowing Christ who was actually perfect.

A lamb “without blemish” is not the same as a perfect lamb. Depending on how you feel about the notion ideal forms a perfect lamb may even be a nonsensical concept. Not only would a perfect lamb have to free from whatever sort of defect was considered a “blemish,” it would also have to have perfect proportions, symmetry et cetera. It does not follow that a lamb without blemish is a “perfect lamb” and it isn’t obvious that a perfect lamb can even be thought to exist.