In general, Matthew Arnold was not a believer in the power of religious faith. He actually felt that religion was a non-entity in the world around him, and as a result, mankind was rather lost, and adrift in the world, because they didn't have a unifying force to bring them together, as religion used to do. He felt that religion wasn't too practical in their world, but that people did need SOMETHING to help them to unite and have a common purpose and goal. In "Dover Beach," the only flare of hope or light that we get is in the last stanza where he implores his love "to be true to one another." He seems to be indicating that in a world devoid of faith (something he says is "withdrawing" and "retreating," leaving the world exposed and "naked"), that loving each other is the only possible solution. Being true to one another, or, loving, trusting, and never betraying each other, is something that they can have that will help them as darkness takes over the world.
When Arnold refers to faith in this poem, he could be referring specifically to religious faith, and how he feels it isn't really found in the world today, or, just any sort of unifying force, like culture, or morals, that brings people together. Either way, he feels that it is gone. People are left without "love, nor light,/Nor certitude, nor peace, nor help for pain." They are adrift and aimless, without hope or direction. His solution, a rather desparate and feeble cry for his love to be true to him, seems inadequate in the face of the "clashing armies" on the "darkling plain" that threaten to overwhelm the world with its darkness.
I hope that those thoughts helped; good luck!
To me, the only point that the author of this poem is making about religion is that the world has come to lack religion. He is saying that religion no longer binds the world together.
You can see this in the third stanza. He says that religion, like the ocean, used to connect the world together. Now, however, it no longer does so. In its place is left nothing but emptiness and the "naked shingles of the world."
While Arnold himself did not believe in Christianity, he did feel that people needed some kind of religious feeling. And he feels that the loss of faith in Christianity has taken something from the world.