This really is a place where looking at the original languages can improve our understanding. We can set aside pisteo for a moment and look only at the word used for "believe not" which is apeitheo and is very telling.Just because you haven’t seen it doesn’t mean it it isn’t true isn’t that literally what we believe in as Christians? I have never seen or experienced healing but I believe anyways
to butcher the word would be to cut or take away from it but how can you do that in the original text there is nothing to cut or take away from it’s the original
This word is not merely failing to believe but an active attitude of being unpersuadeable. It is an active disbelief which is why it is also translated as disobedient.
In both belief and disbelief within the verse the words are active. Believe is have a deep, abiding trust and believe not is willfully resist efforts to convince.
It highlights that man apart from God is not neutral but in active rebellion against God.