2 We are well aware that neither the speech of men nor the analogy of human nature can give us a full insight into the things of God. The ineffable cannot submit to the bounds and limits of definition; that which is spiritual is distinct from every class or instance of bodily things. Yet, since our subject is that of heavenly natures, we must employ ordinary natures and ordinary speech as our means of expressing what our mind apprehends; a means no doubt unworthy of the majesty of God, but forced upon us by feebleness of our intellect, which can use only our own circumstances and our own words to convey to others our perceptions and our conclusions. This truth has been enforced already in the first book, but is now repeated in order that, in any analogies from human affairs which we adduce, we may not be supposed to think of God as resembling embodied natures, or to compare spiritual Beings with our passible selves, but rather be regarded as advancing the outward appearance of visible things as a clue to the inward meaning of things invisible.
Source: On the Trinity (New Advent)