10 Now the contradiction of fools always serves to prove their folly, because with regard to the faults which they contrive by the devices of an unwise or crooked understanding against the truth, while the latter remains unshaken and immovable the things which are opposed to it must needs be regarded as false and foolish. For heretics in their attempt to deceive others by the words, I and the Father are one, that there might not be acknowledged in them the unity and like essence of deity, but only a oneness arising from mutual love and an agreement of wills— these heretics, I say, have brought forward an instance of that unity, as we have shown above, even from the words of our Lord, That they all may be one, as Thou Father art in Me, and I in You, that they also may be in Us. Every man is outside the promises of the Gospel who is outside the faith in them, and by the guilt of an evil understanding has lost all simple hope.
For to know not what you believe demands not so much excuse as a reward, for the greatest service of faith is to hope for that which you know not. But it is the madness of most consummate wickedness either not to believe things which are understood or to have corrupted the sense in which one believes.
Source: On the Trinity (New Advent)