30 These illustrations, I repeat, must only be used as aids to apprehension of the faith, not as standards of comparison for the Divine majesty. Our method is that of using bodily instances as a clue to the invisible. Reverence and reason justify us in using such help, which we find used in God's witness to Himself, while yet we do not aspire to find a parallel to the nature of God. But the minds of simple believers have been distressed by the mad heretical objection that it is wrong to accept a doctrine concerning God which needs, in order to become intelligible, the help of bodily analogies. And therefore, in accordance with that word of our Lord which we have already cited, That which is born of the flesh is flesh, but that which is born of the Spirit is Spirit, we have thought it expedient, since God is Spirit, to give to these comparisons a certain place in our argument. By so doing we shall avert from God the charge that He has deceived us in using these analogies; showing, as we have done, that such illustrations from the nature of His creatures enable us to grasp the meaning of God's self-revelation to us.
Source: On the Trinity (New Advent)