Back to de Lubac. The dogmatic requirement here is that grace is a gift that is unowed to the existing person, qua simply human. This is a requirement of the faith. Challenge it and we are no longer theologians. (No longer Catholic theologians, that is.) Why not? Because theology does not prove the mysteries of faith; it embraces them. It may prove the preambles of faith, but not the mysteries. These are its starting point. Therefore, all theologians must hold that grace is unowed to the existing person, qua simply human.
What does it mean to be “unowed”? It means that it is the opposite of what is owed. Now, that is owed to a creature which is required so that its (the creature’s) existence is intelligible, meaningful, wisely brought about.
Principled theology, as it developed through time, calls such a requirement “debitum naturae,” a thing due to a nature. This requirement is hypothetical. It rests on the existence of the nature, in this case the human person. But as we also confess – and can prove rationally – the world is created freely. God need not have created man. So, the requirement in debitum naturae runs thus: IF there is to be a man (freely created), THEN such and such are required so that he be wisely made.