Of course it's a form of language (well, communication), but it's not limited to expressions of one's beliefs. Art, graphic or otherwise, explores ideas, and real artists aren't afraid to explore ideas they don't hold. For instance, JMS (a staunch atheist) penned one of my favorite quotes: "Faith and reason are the shoes on your feet. You can go further with both than just one."
What's more, inspiration and reference don't even necessarily come from ideas, ESPECIALLY in the visual arts. A Satanic painter might really like the composition of the Chapel's ceiling and be inspired to reference it without intending any theological commentary.
The complex interweaving of self-expression, commentary, and simple aesthetics is precisely what makes art so fascinating.