On The Holy Trinity by St. Augustine
Chapter 2.--In What Manner This Work Proposes to Discourse Concerning the Trinity.
4. Wherefore, our Lord God helping, we will undertake to render, as far as we are able, that very account which they so importunately demand: viz., that the Trinity is the one and only and true God, and also how the Father, the Son, and the Holy Spirit are rightly said, believed, understood, to be of one and the same substance or essence; in such wise that they may not fancy themselves mocked by excuses on our part, but may find by actual trial, both that the highest good is that which is discerned by the most purified minds, and that for this reason it cannot be discerned or understood by themselves, because the eye of the human mind, being weak, is dazzled in that so transcendent light, unless it be invigorated by the nourishment of the righteousness of faith. First, however, we must demonstrate, according to the authority of the Holy Scriptures, whether the faith be so. Then, if God be willing and aid us, we may perhaps at least so far serve these talkative arguers -- more puffed up than capable, and therefore laboring under the more dangerous disease -- as to enable them to find something which they are not able to doubt, that so, in that case where they cannot find the like, they may be led to lay the fault to their own minds, rather than to the truth itself or to our reasonings; and thus, if there be anything in them of either love or fear towards God, they may return and begin from faith in due order: perceiving at length how healthful a medicine has been provided for the faithful in the holy Church, whereby a heedful piety, healing the feebleness of the mind, may render it able to perceive the unchangeable truth, and hinder it from falling headlong, through disorderly rashness, into pestilent and false opinion. Neither will I myself shrink from inquiry, if I am anywhere in doubt; nor be ashamed to learn, if I am anywhere in error.