In the post below, I mentioned parenthetically that a key question is, Do we obey out of love, or love out of obedience?
There are a couple of ways of understanding this question. One is to take it as a question about us, about what you and I and the other people actually do. This is probably the natural interpretation, given the way the question is phrased.
Another way, though, is to understand it as a question about love and obedience, about which (if either) is the foundation of the moral order. It's this understanding that, to my mind, makes it a key question.
It may not seem all that important. After all, we ought to both love and obey God, and the Gospels make it clear that we can't properly do one without the other.
But what we believe about the foundation of the moral order amounts to what we believe about the foundation of our relationship with God, which amounts to what we believe about our fundamental nature -- and God's nature as well.
If obedience were the foundation of the moral order, then our relationship with God would be fundamentally one of lawgiver-lawkeeper. Whatever love there may be between God and ourselves would be a non-essential addition to this fundamental relationship. We would necessarily remain apart from God, kept at a distance by the law that exists between us. God would mediate Himself through His commandments. This mediation, and this distance, would persist after the Last Judgment, since it would be the foundation of our relationship with God. Whatever He is in Himself, He would be Revealed to us as a legislator; we could never draw closer to him than the stone tablets He sends down the mountain to us.
Furthermore, the lawgiver-lawkeeper relationship would be the light by which we understand our relationships with each other. The more commandments I give others, the more God-like I become. Which commandments I should give to become more God-like would be hard to say, since there wouldn't be much in the way of a necessary connection between God's nature and His commandments.
And where commandments are silent? Where they are incomplete or not spelled out or don't give complete guidance? There morality, depending on obedience for its foundation, could not exist.
Now yes, the first and greatest commandment is to love God, so doesn't the case of obedience being the foundation of the moral order turn into the case of love being its foundation in one easy step?
No. You don't have one foundation on top of another; avoiding that is sort of the whole point of talking about foundations.
More substantially, love is fundamentally not a matter of obedience. The Greatest Commandment notwithstanding, you can't command others to love, because love cannot be commanded. In St. Thomas's formulation, the theological virtue of charity is friendship with God, and while God can command us to act like His friends, He cannot command us to be His friends.
Among the other consequences of wrongly believing that obedience is the foundation of the moral order, then, is an inadequate understanding of love as purely a matter of action and not of will, as a matter of doing and not of being.