18 And you, whose warmth of faith and passion for a truth unknown to the world and its philosophers shall prompt to read me, must remember to eschew the feeble and baseless conjectures of earthly minds, and in devout willingness to learn must break down the barriers of prejudice and half-knowledge. The new faculties of the regenerate intellect are needed; each must have his understanding enlightened by the heavenly gift imparted to the soul. First he must take his stand upon the sure ground [substantia = ὑποστάσει] of God, as holy Jeremiah says, that since he is to hear about that nature [substantia] he may expand his thoughts till they are worthy of the theme, not fixing some arbitrary standard for himself, but judging as of infinity.
And again, though he be aware that he is partaker of the Divine nature, as the holy apostle Peter says in his second Epistle, yet he must not measure the Divine nature by the limitations of his own, but gauge God's assertions concerning Himself by the scale of His own glorious self-revelation. For he is the best student who does not read his thoughts into the book, but lets it reveal its own; who draws from it its sense, and does not import his own into it, nor force upon its words a meaning which he had determined was the right one before he opened its pages.
Since then we are to discourse of the things of God, let us assume that God has full knowledge of Himself, and bow with humble reverence to His words. For He Whom we can only know through His own utterances is the fitting witness concerning Himself.
Source: On the Trinity (New Advent)