6 It is the Father to Whom all existence owes its origin. In Christ and through Christ He is the source of all. In contrast to all else He is self-existent. He does not draw His being from without, but possesses it from Himself and in Himself. He is infinite, for nothing contains Him and He contains all things; He is eternally unconditioned by space, for He is illimitable; eternally anterior to time, for time is His creation. Let imagination range to what you may suppose is God's utmost limit, and you will find Him present there; strain as you will there is always a further horizon towards which to strain.
Infinity is His property, just as the power of making such effort is yours. Words will fail you, but His being will not be circumscribed. Or again, turn back the pages of history, and you will find Him ever present; should numbers fail to express the antiquity to which you have penetrated, yet God's eternity is not diminished. Gird up your intellect to comprehend Him as a whole; He eludes you. God, as a whole, has left something within your grasp, but this something is inextricably involved in His entirety.
Thus you have missed the whole, since it is only a part which remains in your hands; nay, not even a part, for you are dealing with a whole which you have failed to divide. For a part implies division, a whole is undivided, and God is everywhere and wholly present wherever He is. Reason, therefore, cannot cope with Him, since no point of contemplation can be found outside Himself and since eternity is eternally His. This is a true statement of the mystery of that unfathomable nature which is expressed by the Name 'Father:' God invisible, ineffable, infinite.
Let us confess by our silence that words cannot describe Him; let sense admit that it is foiled in the attempt to apprehend, and reason in the effort to define. Yet He has, as we said, in 'Father' a name to indicate His nature; He is a Father unconditioned. He does not, as men do, receive the power of paternity from an external source. He is unbegotten, everlasting, inherently eternal. To the Son only is He known, for no one knows the Father save the Son and him to whom the Son wills to reveal Him, nor yet the Son save the Father. Each has perfect and complete knowledge of the Other. Therefore, since no one knows the Father save the Son, let our thoughts of the Father be at one with the thoughts of the Son, the only faithful Witness, Who reveals Him to us.
Source: On the Trinity (New Advent)