Is it ever acceptable for a Catholic to enter the "chapels" of the protestant heretics, the eastern schismatics, the Jєωs, the Islamic infidels, etc.? I would say absolutely not for their "worship services" (but I have heard it argued that Catholics can attend weddings & funerals so long as they do not participate beyond mere presence), but what about other things? What if a Catholic electrician is hired to do work on a Mosque? What if a Catholic painter is hired to paint the interior of a Methodist "chapel"?