In his commentary on the Treatise on the Trinity, Gerrigou-Lagrange says that we should not speak of the scriptures proving that God is triune but we should rather speak of the testimony of the scriptures about the triune God. While it would be going too far to say that we should entirely replace talk of proving things with talk of giving the scriptural witness of things, we should at least be sensitive to the reality that the greatest mysteries of the faith – that is, those things that most deserve to be called revealed - are things for which we more have testimony than proof.
The sharpest difference between rational proof and testimony is in the way they are common to many. Rational proof is common to many in its very principles, being given either to everyone or to those with the right perquisites (what STA calls “self-evident to the wise”) but testimony is common to many only by the testimony given. The principle of the truth of the testimony is hidden from many, who only have access to it through testimony. In reveling himself to the world, the orthodox Christian claim goes, God has chosen to rely on testimony more than rational proof, which is why he has given faith pride of place over knowledge. The value of faith, however, is relative; the absolute preference is for testimony over proof. Why this preference?
The strongest argument one could make for this preference is that is logically necessary, and there can be a decent argument for it- if revelation is most fundamentally the words of a God-man to the world then it is impossible that revelation come except by testimony, since what is seen or known by a God-man, properly and as such, simply cannot be seen or known by a world of non God-men. The desire to do away with testimony altogether is would be the desire that revelation not come through Christ. One can posit all the miracles and wonders that they want – such things can never close the gap that makes the testimony necessary. Given the necessity of testimony, there is even an argument to be made for the limitation of miracles, since miracles are more given in the line of evidence than the line of testimony, and if these two are opposed and the latter is logically necessary, there would be a reason not to do absolutely as many miracles as possible. The one with evidence is even at a certain disadvantage, since there is the danger in evidence taking the place of testimony.