In the view of other people, even if they don't admit it, a person who says things confidently as the truth, which others do not think are the truth and do not think can be true, is someone who is telling lies or spreading falsehood, and someone who is spreading falsehood or telling lies, even if they don't think those are lies (like a schizophrenic for example), is considered a Liar, a Lying Person, someone who Lies, someone telling Lies, A Liar, Dishonest, Untruthful, Not in Accord with the Truth, someone spreading falsehood, one who is deceiving (even not intentionally).
So it is like how some people say "I was taken under the Earth by a bunch of lizard people, there are lizard people under the Earth, they took me and they showed me radio devices that can mind control others, and they wanted me to join them, but I said no, so they transported me back to my bed". Now, most people would think these are not true things being talked about, that this person is basically telling lies, and that they may have even experienced such, but that such is not true, it is not real, it is not in accord with reality, it is false, falsehood, lies, there are no lizard people, etc.
More than that, Christians say things which they don't know, they say that the events of the Gospels are true, but they never saw these things themselves, and they say that it is true that to say Jesus is God and the Son of God will save them from their sins, and they say other things, which they don't know to be true or verified at all really, but they say it is the truth, and they insist upon it, while others do not consider this to be the truth at all, and thus they consider them (even if they don't admit to it) liars telling lies, spreading falsehood, misleading people, leading people to harm and error by their statements which are untruths that can never be true, and that to say things are so which you can't possibly know for certain are so, makes one a dishonest speaker, one who is untrustworthy, a person who says things that they don't know are true "are true" without knowing or being able to clearly demonstrate it as so, and so they are dishonest.
Christians, in the opinion of others, thus live a lie, they wake up lying, they go to sleep lying, their whole worldview is made up of lies, they are completely false in the view of others. Likewise, Christians believe everyone else is deceived by the "God of this World", because they think those who say God can not ever be incarnate as a man, Jesus, are the real liars.
So each thinks the other is a dishonest liar telling lies.