J. L. Schellenberg is an atheistic philosopher who has advocated a kind of secular religion. The following is based on my imperfect recollection of his work from a couple of years ago and some cursory research.
Essentially, we are very young as a species from the perspective of deep time (think millions of years), so our best religious ideas might still be ahead of us. Based on that premise, “ultimists” would explore different contemporary conceptions of ultimate reality without committing to any of them.
Schellenberg does think he has good reasons to believe that there may be an ultimate reality which (a) everything that exists depends on, (b) is the greatest thing possible, and (c) benefits humanity when they act on the premise that it exists. These reasons are adaptations of the classic arguments for God’s existence, although he thinks the original versions fail, and his reformulations are very different from the originals. However, he believes that the problems of evil and divine hiddenness show that the Christian conception of God cannot exist, so that can’t be the ultimate reality he is in search of.
I would say Schellenberg is at least as interesting an author as many Christian authors writing today, so he’s worth reading if you have a serious interest in philosophy of religion.