While most religions claim to be the one true religion, the Bible doesn't actually say Christianity is the true religion. It takes a much deeper approach. See, Christianity, at least as the world perceives it, is just belief in some form of Jesus. But some people, by this logic, could be genuinely considered Christians, as they believe Jesus died and rose for their sins, yet by their works, they deny Him.
James talks about religion in his letter. He gives three elements of true religion: Control of the tongue, care for the helpless, and to keep unspotted from the world.
At first, this can be quite worrying. James himself says that taming the tongue is harder than taming an animal. And who among us is effectively sinless? Are you always charitable? Do you never fall for worldly things? Of course James is not telling us we're either perfect or we're condemned. But he is setting us a standard. If anyone thinks he is religious, yet does not control his tongue, his religion is useless.
Anyone can say they're a Christian. "I was raised Christian", "I go to church", "I wear a cross". My brethren, my dog wears a cross. I can assure you, he is far too derpy to comprehend the faith. But what about you? Do you live the faith you profess? Compare your works to the Bible. Do they line up? Now look at the world. Do they resemble those? Sadly, many Christians can only answer the latter. Are you one of them? If so, change. Otherwise, under the inspiration of God, James tells you your religion is worthless.