It is impossible to attain to the knowledge of the Trinity by natural reason. For...man cannot obtain the knowledge of God by natural reason except from creatures. Now creatures lead us to the knowledge of God, as effects do to their cause. Accordingly, by natural reason we can know of God that only which of necessity belongs to Him as the principle of things, and we have cited this fundamental principle in treating of God.... Now, the creative power of God is common to the whole Trinity; and hence it belongs to the unity of the essence, and not to the distinction of the persons. Therefore, by natural reason we can know what belongs to the unity of the essence, but not what belongs to the distinction of the persons (ST I Q32 A1).Since some things can only be known by faith, St. Thomas says that to attempt to prove them by means of reason detracts from "the dignity of faith" (ibid.), is ultimately futile, and actually dishonors the faith. It detracts from the faith by reducing it to a matter of reason; it is futile in that such efforts can never be successful precisely because matters of faith are above the capacity of reason; and it dishonors the faith,
[f]or when anyone in the endeavor to prove the faith brings forward reasons which are not cogent, he falls under the ridicule of the unbelievers: since they suppose that we stand upon such reasons, and that we believe on such grounds (ibid).Thus, although the effort may be well-meaning, the effect is exactly contrary to was intended. Rather than attempting to prove such things, St. Thomas recommends two courses of action: with those who accept the authority of Sacred Tradition, Scripture, and the Church, we need do no more than appeal to that authority; with others, the most that we can do is to show that what we believe by faith is not impossible.