7 It is easier for me to feel this concerning the Father than to say it. I am well aware that no words are adequate to describe His attributes. We must feel that He is invisible, incomprehensible, eternal. But to say that He is self-existent and self-originating and self-sustained, that He is invisible and incomprehensible and immortal; all this is an acknowledgment of His glory, a hint of our meaning, a sketch of our thoughts, but speech is powerless to tell us what God is, words cannot express the reality.
You hear that He is self-existent; human reason cannot explain such independence. We can find objects which uphold, and objects which are upheld, but that which thus exists is obviously distinct from that which is the cause of its existence. Again, if you hear that He is self-originating, no instance can be found in which the giver of the gift of life is identical with the life that is given. If you hear that He is immortal, then there is something which does not spring from Him and with which He has, by His very nature, no contact; and, indeed, death is not the only thing which this word 'immortal' claims as independent of God. If you hear that He is incomprehensible, that is as much as to say that He is non-existent, since contact with Him is impossible.
If you say that He is invisible, a being that does not visibly exist cannot be sure of its own existence. Thus our confession of God fails through the defects of language; the best combination of words we can devise cannot indicate the reality and the greatness of God. The perfect knowledge of God is so to know Him that we are sure we must not be ignorant of Him, yet cannot describe Him. We must believe, must apprehend, must worship; and such acts of devotion must stand in lieu of definition.
Source: On the Trinity (New Advent)