Delusion is neither a Christian nor a non-Christian matter. Delusion is simply believing something that isn't true despite overwhelming evidence. I don't believe there is a specific "Christian" perspective on it.
I'm going to try very hard to answer the rest of this objectively and not get on a soap box (even though it goes against my obnoxious nature).
Let's start with a definition of a "delusion", straight from Wikipedia.
A delusion is a belief held with strong conviction despite superior
evidence. Unlike hallucinations, delusions are always pathological
(the result of an illness or illness process). As a pathology, it
is distinct from a belief based on false or incomplete information,
dogma, poor memory, illusion, or other effects of perception.
As long as we are limited finite beings, a certain amount of uncertainty exists in determining what is "real" and what is not. As long as we are finite, we can't know and prove everything.
As long as we're not omniscient then the best we can do it to try to interpret the evidence in such a way that it makes sense to us.
Here's an example an atheist and non-atheist can relate to:
For years, scientists thought that atoms were the smallest level of matter, until they realized that atoms are made up of proton, neutrons, and electrons. Then came quantum physics, which indicates that at that level, reality is a lot more strange and complex than ordinary physics. Does this mean that the scientists that didn't understand quantum physics were "delusional"? No, they simply didn't have all the facts. We still don't.
Going back to the "superior evidence" portion of the definition, there is plenty of evidence both for and against the existence of God, creationism, theism, the Bible as God's word, and a whole host of other beliefs that separate us. The problem isn't the evidence itself in many cases, it's how our own personal biases and our own presuppositions color how we interpret the evidence. We don't have conclusive evidence for or against Christianity.
I came from the atheist viewpoint to be a very firm believer in the most conservative young-earth view of Christianity based on examining the evidence. I was attempting to show my mom how dumb Christianity is. My own bias was against Christianity. I don't say this to convince you or anyone else, but I do say it to back up the fact that I have examined the evidence, and I would argue with anyone that the evidence is far from conclusive either way.
I came to my position not because the evidence for God was so overwhelming, but because I learned through years of research that the evidence against His existence wasn't as strong as I'd believed, and an open mind led me to believe, personally, that the evidence for Him has fewer unresolved assumptions and problematic discrepancies.
Therefore, Christianity is not delusional, nor is atheism. Neither one fits the textbook definition of "against overwhelming evidence". There's a big difference between "there's overwhelming evidence" and "I'm only aware of some of the evidence and reject out-of hand the evidence that doesn't support my viewpoint."
From the note it looks like I wen the wrong route and am not answering your question as you intended, but I think the above still applies.
In 2 Thessalonians 2:11, (KJV) we read:
And for this cause God shall send them strong delusion, that they
should believe a lie:
This delusion simply refers to a falsehood - a false belief, still meeting the textbook definition above. This is just a specific delusion being spoken of in this case. it doesn't nullify or establish the idea of "delusion" still meeting the normal understanding of the word.