Is life more satisfying as an atheist or religionist?

[Belacqua]

My case is simply that it is highly arguable that the best cultural works are religious.

I'd argue it. The majority of great cultural works in European and Asian history are religious. 

  Both Shakespeare and Scorsese are secular and better than the majority of medieval Christian texts.   

Ah, but you've put your thumb on the scale there. Shakespeare is greater than the majority of medieval Christian texts. Because the majority of any genre will be average. He's also greater than the majority of Elizabethan secular texts. But is he greater than Dante? Is he greater than Augustine's Confessions? Bach? Michelangelo? When we're talking about the real greats, it's difficult to make a confident ranking, but his peers will be nearly all religious. 

Would you not count Proust as secular?  A Catholic milieu, sure, but the text itself isn't Catholic.  I would think everything not explicitly religious is secular.

As I said, he could only have come from a Catholic culture. His book is not, itself, on the topic of religion. But to read it in a careful and sympathetic manner requires a sympathy for that culture. When he said that he wanted his novel to be a cathedral, this is not only about size and complexity. This is true for all the modern French greats -- Baudelaire, though dark and even Satanic, is rooted in Catholicism. The Decadents intentionally invert religion. Gide, Bataille, and many others, are either Catholic or working as its opposition. From that era, the only explicitly and intentionally secular movement in the arts I can think of offhand would be the Impressionists. But remember that Van Gogh only turned to painting because he failed as a preacher, and his passion for the world (it appears from his letters) is rooted in his sense of the sacred. So even the purely visual arts can't escape a base in religious history.