if any of you are formerly religious and were convinced to stop being religious by a person who once belonged to your religion, maybe you have a useful anecdote for how they were able to arouse doubt in your beliefs without insulting your intelligence.

I'm Catholic, was before my radicalization and still am, but obviously had a lot of homophobia and transphobia to unlearn. More relevant to this question though, the way I thought about Marxists before I radicalized had to change, and while I still don't use the label Marxist I'd say I use Marxist thought to analyze basically all social phenomena. I think the critical thing for me was understanding that the conflict between Communist states in history (and to some extent, the French Revolution) and religion didn't stem out of the left's hatred of religion, but out of the Church's resistance to social change. I had to understand that the Church was kind of a load-bearing structural member of the social order and legitimized all the oppressive institutions, so naturally the radicals who aimed to abolish said social order needed to take measures against religious institutions. If I could talk to myself before I was politically conscious, I'd tell him that he should consider that the way things were arranged in the Russian Empire, Feudal China, France, etc. the only possible way to get the masses to throw off their shackles was to attack corrupt religious institutions that were literally conspiring and collaborating with the secular states to keep peasants down. That the violent measures that I was taught about were used as a last resort, and were only a reflection of the violence that the overthrown institutions had used liberally for centuries.

As to how this pertains to anti-authoritarian leftists, I think you could modify this argument a little bit, and apply it to whatever they think authoritarianism is. "Tankies" don't want to use authoritarian tactics because they hate freedom, or because they want to restrict individuality. They want to use those tactics to achieve the same political goals as all leftists: to abolish capitalism.