Why is gatsby's love for daisy described in religious terms?

Because it's like he was set on finding this "holy grail". And as most know, the holy grail is not exactly a real thing unless you're thinking of it back to arthurian literature, but rather a word used to describe something that is of the utmost importance to them, if that makes sense. For Gatsby, his holy grail was Daisy / having her love. The holy grail is a religious thing. It's just like comparing them together.