It will really depend on the story itself, the tone of it, how you present the story. The Lord of the Rings, for example, has a 'diverse' fantasy cast and no religion.
It depends on the contract you set up with the reader.
You may have readers asking if your characters believe in God. (I had readers that asked me that.) This is a normal thing to wonder about, as a reader. Clearly, since you have no religion mentioned in your story at the moment, it is an aspect of diversity that you aren't addressing with this book. At least, not yet. If readers aren't asking about religion, and if you feel your book has enough focus on the elements you wish to address without bringing in religion, then don't sweat it. But if readers are asking about it, then something in the book is causing them to wonder. In that case, you have a few choices, not limited to:
~ Set the contract so that you are making it clear religion isn't part of the story, at all. It will be completely absent. Have a character say something early on, about the world not caring about religion anymore, or something like this. If a character makes it clear for the reader to not expect religion, the reader is far more likely to accept those terms for the story.
~ If religion is still a part of the culture, but not within your story, you could nod to the reader with a simple few lines early on--Perhaps they are passing a mosque/temple/church/etc and one character off-handedly says 'I haven't gone in years.' I think that's enough to say to the reader, 'yeah, there's religion in the world but not in the story.'
~ Decide for yourself if your characters are religious, and if so how that shapes them as individuals. Maybe one is agnostic, and only prays in times of stress, for example, and maybe another is superstitious and maybe another is a militant atheist who becomes angry if the topic comes up. If you want to add religion as a sub-thread throughout your story, decide these things and in revision drafts, allow the religious beliefs to come through.
In my opinion, from what I have seen, most novice drafts (including my own, but not all) can benefit from more depth which means subplots or deeper characterization. It's up to you to decide where to go with this. But stories are not real life, and they don't chronicle real life. They are reflections of real life. We experience them within our minds, which are not the same as our external worlds. We want to understand our own lives, their messy complexity, but when we try to understand our lives we do it a couple pieces at a time. You are allowed to have whatever cast you like and whatever elements you like. (For example, have you included the diversity of dietary beliefs and behaviors among your characters? The diversity of wealth and opportunity? The diversity of educational opportunity and attainment? Political beliefs? Health issues? Ages? And so on.)
Although the readers' experience is of supreme importance, and you do need to pay attention to it, the important thing up front is to define the contract to the terms you are offering. And write a good story.