So far I have shown that God is one, that he is the Absolute, that he is the primary reality that underlies all things, and that all that exists, takes shape, and endures in the universe is from him. It follows then that he is infinite.
Just below [Section 32] I will show that many phenomena in the created universe enable human reason to see the infinity of God. Yet although the human mind can use those phenomena to support an acknowledgment that the first entity or primary being is infinite, still it cannot come to know what the Infinite is like. The only way it can define the Infinite is to say that it is utterly without limits and self-sufficient, and is therefore the absolute and only substance. Because substance has no attributes without form, it is also the absolute and only form. What substance and form are in their full infinity, however, is not apparent. The human mind itself, even the most highly analytical and elevated mind, is finite; it cannot be rid of its own limitations. It will never have the capacity to see the infinity of God as it truly is, or God as he truly is. It can see God in a shadow from behind, as Moses was told to do when he begged to see God. He was put in a crevice in the rock and saw God’s back (Exodus 33:20–23). “God’s back” has as a general meaning the phenomena visible in the world and has as a specific meaning the things that are comprehensible in the Word.
It is obviously pointless then to aim to find out what God is like in his own underlying reality or in his own substance. It is enough to acknowledge him from finite, created things, in which he is infinitely present.
The person who goes farther than that could be compared to a fish hauled out into the air; or to a bird put in a vacuum pump, gasping as the air is pumped out, and soon expiring; or to a ship overcome by a storm, no longer responsive to the helm, drifting onto reefs and sandbanks. Something comparable happens to people who want to know the infinity of God from the inside and are not content to acknowledge it from the outside on good evidence.
We read that a philosopher among the ancients threw himself into the sea because in the mental light he had he could not envision or comprehend the eternity of the world. What if he had tried to see the infinity of God?
from True Christianity, Volume 1, Section 28