Is it okay to call someone a fool?
No
“But anyone who says [to a brother], ‘You fool!’ will be subject to the fire of hell.” - Matt 5:22b
Yes
If they are atheists - Psalms 14:1, 53:1
If they trust their own hearts - Prov 28:26
Jesus called people fools (in the classic “do as I say, not as I do” scenarios) - Matt 23:17,19; Luke 11:40, 24:25
Paul did as well - Rom 1:21-22; 1 Cor 15:36; Gal 3:1
There seems to be more weight toward calling people fools than not. There are also 5 ways to call someone foolish (anoétos, aphrón, asunetos, mórainó, and móros) in the NT.
Who does this apply to?
According to John Gill, ‘brother’ in “...whosoever is angry with his brother without a cause...” does not mean
One of the same church state
It means in a large sense any man of any country or nation. We are to be angry with no man.
The greek word adelphos, translated as ‘brother’ actually mean “a brother, member of the same religious community, especially a fellow-christian.”
So it’s a bit disingenuous to claim that it actually means “any man.” I will however agree that it’s better to apply it to everyone, not just those one is religiously associated with, which is what the verse means.