One of the points that Lovejoy makes in the book of that title I mentioned last week is the importance, in the Neo-Platonist conceptions and in the later development of the "chain of being" metaphor, of what he calls the principle of plenitude. This is the underlying notion that everything that can exist must exist, that creation would not be possible at all were it to leave gaps.
The value of this idea for a certain type of theodicy is clear enough.
This caused theological difficulties when these ideas were absorbed into Christianity. I'll quote a bit of what Lovejoy has to say about those difficulties:
"For that conception, when taken over into Christianity, had to be accommodated to very different principles, drawn from other sources, which forbade its literal interpretation; to carry it through to what seemed to be its necessary implications was to be sure of falling into one theological pitfall or another."
The big pitfalls were: determinism on the one hand, antinomianism on the other. Since the principle tells us that things must be as they are, it implies that God had no choice in the issue at hand, He had to create exactly the world he did. Yet to deny freedom to God ... that was an inference whence one had to shrink. Also, since the principle of plenitude tells us that the bad is part of the perfect overall plan, it tends to erase the badness of the bad too thoroughly for comfort, making sin too easily forgivable.
It was Abelard, Lovejoy tells us, who "indiscreetly made manifest both the deterministic and the antinomian implications of principles which nearly everyone accepted. It was one of the heresies charged against Abelard by Bernard of Clairvaux that he taught 'that God ought not to prevent evils, since by His beneficence everything that happens does so in the best possible manner.'"
This conflict, between acceptance of the principle of plenitude and rejection of its principles, "which had in the time of Abelard taken the form of open controversy, continued to manifest itself in the form of an inner opposition of tendencies in the minds of individual thinkers."