Beliefs inform actions, so it’s impossible that a believer never affects someone. Sooner or later the believer will do something damaging.
Procreating and passing on the delusion. Harassing non traditional sexualities or lifestyles. Preaching. Preventing their children from following their own love or their own vocation because it goes contrary to their religious tenets. Supporting “charities” that take the help hostage until the helped listen to the sermon or embrace the faith. Teaching that the only source of morality is religion, and specifically theirs.
Also, leaving someone believe something intrinsically damaging and doing nothing is an asshole move.
Of course it doesn’t benefit you but them. But maybe helping others is against your own beliefs.