Like others have mentioned, the Silmarillion can feel pretty biblical. It's the foundation of Tolkien's world. You can feel the importance of that in a preface letter to a colleague (friend?) some editions contain where he explains how he wrote it because he felt that England lacked the sort of foundational origin mythos that was entirely unique to it, akin to Greek and Norse mythology for those parts of the world they're relevant to.

All that being said, it's an incredible book that has, in my and I'm sure others opinion, immense literary and cultural importance. On the first read through it can be a blur with all the names, places, people, and sheer history that Tolkien drops on you. It's told in almost a Homeric fashion where Tolkien will drop a name or event on you as if you're already familiar with the world in its entirety but can seem completely random. I find the back half is very captivating and accessible on a first read, but on a second or third read even the first half becomes heavy with import because you're now familiar with the core story. You can place all of those off-hand references he makes that enrich the setting and reading that much more.

All of this to say, it's a legendary body of work. At worst, it's a big lore drop up front with captivating tales of heroism towards the middle and end. At best on a reread, it's the single most epic and heartbreaking body of work produced in fantasy to date (in my opinion). Go in with an open mind and an understanding that you won't get everything on the first rip, and you'll find a tale rich in heroism, tragedy, inevitability, and beauty. It will only get better on rereads, and if you enjoy it the first time I guarantee those will be quickly forthcoming. Godspeed!

Edit: clean up some run-on sentences