You are quite correct, secular socialists, communists, and various other revolutionary materialists have NO PROBLEM AT ALL using religious building to store arms and explosives, recruit members, hold meetings, etc.
And you think that, conversely, a Mosque or other religious building will have NO PROBLEM AT ALL with a secular organization using them to store arms and explosives, recruit members, and hold meetings?
Fatah is an Islamic terrorist organization. In no way are they a “secular” organization. Their name means expansion through jihad, the word was traditionally used to describe the expansion of the Islamic word by the sword in the first few hundred years.
It is as if a group formed that called themselves “the Crusaders” and recruited members from and stored arms and explosives inside a Church; and yet you insist they are secular.
Delusional.