I'm not writing anything new here, nothing that hasn't been thought of before. But I think it's significant when a person realizes for herself what it really means to believe in a God who is infinite and creates and maintains a universe with infinite space and time.
The feeling is similar to the one that I get--fear--when I think about the fact that I must one day cease to live as I know it and enter a new form of existence--or none at all.
As a Christian, I am expected to insist without doubt that what follows life is heaven and eternal dwelling with God. However, to do this myself would constitute intellectual dishonesty. Thus, I align myself with these beliefs but cannot eradicate completely the abysmally discomforting uncertainty that must lurk behind this most comforting answer to this most fundamental of questions.
Thus...the fear. Although fear is a woefully inadequate word for this feeling. "Abject terror" comes a little closer to the feeling. "Awestruck horror"..."dumbfounded wonderment"...both somewhat helpful approximations.
What to do with the fear? I don't actually know. Probably something like...adopt a humble attitude. Spend my short time alive wisely. Keep a constant healthy respect for the infinite creator. Be open to the possibility that there is life in some distant corner of the universe, unspeakable fathoms hence.