Aquinas affirms that “Good is what all desire” (cf. Aristotle, Ethics 1.1; Aquinas, Commentary on Aristotle’s Ethics 1, 9 and 11). However, unlike modern theories of desire, Aquinas clarified that “When we say that good is what all desire, it is not to be understood that every kind of good thing is desired by all, but that whatever is desired has the nature of good” (ST I 6,2 ad 2).
So, those not in communion with Christ can know the first principles of moral actions and act upon them. But if every person desires the good, and God is the ultimate good – doesn’t it follow that everyone should also desire to be in communion with God?