21 Human judgment must not pass its sentence upon God. Our nature is not such that it can lift itself by its own forces to the contemplation of heavenly things. We must learn from God what we are to think of God; we have no source of knowledge but Himself. You may be as carefully trained as you will in secular philosophy; you may have lived a life of righteousness. All this will contribute to your mental satisfaction, but it will not help you to know God. Moses was adopted as the son of the queen, and instructed in all the wisdom of the Egyptians; he had, moreover, out of loyalty to his race avenged the wrong of the Hebrew by slaying the Egyptian, and yet he knew not the God Who had blessed his fathers.
For when he left Egypt through fear of the discovery of his deed, and was living as a shepherd in the land of Midian, he saw a fire in the bush, and the bush unconsumed. Then it was that he heard the voice of God, and asked His name, and learned His nature. Of all this he could have known nothing except through God Himself. And we, in like manner, must confine ourselves, in whatever we say of God, to the terms in which He has spoken to our understanding concerning Himself.
Source: On the Trinity (New Advent)