Our modern, Western culture is the strangest of them all. It's understandable when followers of other religions call Christians "delusional". Muslims, for example, did not make up Islam. They're following a religion that's been around for approximately 1400 years. But in a world of "live your truth", how can your average Relativist call anyone delusional?
As a Christian, I do not make up my own beliefs. At the very least, not intentionally. Obviously, it's only natural, given that I am a fallible human being, to attempt to fill in gaps in my knowledge. Just like if I tell you about my dog. If you've never seen my dog before, you probably have your own "default" dog in your head right now. For me, that's usually a brown labrador-type, which is strange, since I've never owned a labrador. For you, it could be a pug, a dalmation, a chihuahua etc. None of these are accurate; you have naturally filled in your own gaps. These are inaccurate. Unless you've seen him before, you probably cannot picture my dog.
But even with your inaccurate picture of my dog, you cannot be called delusional for acknowledging that I have one. Why? Well, first of all, because I've told you. But also, because I do have a dog. His name is Stan, he's a golden brown cross breed, nearly 3 years old, and he wears a blue collar. These are all facts. Facts which you can know, because I have told you. Of course, without seeing him, you're still filling in gaps, but the real delusion would be to say that I actually own a 7 year old black hamster who doesn't wear a collar.
Believing in my dog is an example of a received belief. You believe because I have told you to. You did not make it up. Even with the assumptions you made to fill in gaps in your knowledge, you did not make Stan up. He exists. The hamster, on the other hand, is completely made up. If you believe I own a hamster, you are delusional, because for whatever reason, you are making up things that are not true, and treating them as true.
When it comes to most religions, the individual believers are not making much up. They fill in gaps themselves, but they are nevertheless believing what they have received. This is often backed by evidence of some kind. Christianity in particular is based on a rich and deep history, recorded in the Bible, and often corroborated by other sources. Even if you want to claim the Bible is made up, it was not made up by anyone alive, and the evidence in its favor is so strong that if it is made up, it was made up quite convincingly.
But the majority of atheists, while they claim to have no religious beliefs, are living "truths" they made up. If you're "living your truth", as we are often told we should (unless that truth happens to be Christianity, apparently), then you are living no truth at all. Like my 7 year old black hamster, you are making up lies, and believing them, not because you are actually convinced, but simply because you want to.
But let's be honest here; we all know this doesn't work. We can't manifest things out of thin air! We can't alter anything with our minds. Even our very thoughts are often intrusive. Try walking into a psychologists office and telling a PTSD patient "live your truth". Oh ok, great, now the traumatic memories are gone... "Live your truth"? There's no such thing as your truth! You cannot even bite into an apple and decide what it tastes like. So how do you imagine your beliefs about God, life, and death, will have any effect on the reality?
If there is no God, Christians are still believing a lie. One might call that a delusion. But it makes far more sense to believe something someone else made up than to believe something you made up, especially if there is evidence for the other delusion. If, by contrast, Christianity is true, Christians are still going to have to fill in our own gaps, but we are merely attempting to perceive a received truth. You, by contrast, are trying to overcome it! You are deluding yourself, acting as if you are God!
But the thing about self delusions is they tend to catch up with us. If you cross the street without due care and attention, I'm sorry, but a bus is a bus. So ask yourself one question: This "truth" you are living... Is it worth getting hit by God's "bus"?