Well, as always, context is king and stating that "Islam is not a race, so therefore criticizing Islam is not racist" ignores the content of the criticism as well as a lot of social context..

As to the content, we don't really know because OP didn't tell us. He's asking for blanket acceptance of the premise that nobody should be allowed to call him racist for criticising Islam - and I'm not willing to give him that.

I firmly believe that anyone who wants the freedom to say whatever they want has to give the freedom of criticism to their audience. That means, if one cares enough about being called racist that one posts a meme calling those critics stupid, maybe one should instead endeavor to ask and learn why their comments seem racist, rather than dismissing it outright.

As to the context, I am a white American dude and though I try to educate myself on all perspectives, I don't pretend to understand what a Muslim goes through. That said, I do know a few pieces of context that shoot holes in the premise:

1. Racists absolutely use "muslim" to refer to a generalized swath of "brown people" from the middle-east, balkans, and part of eastern europe. Racists often use "muslims" instead of ethnicity because they don't actually know the ethnicity they're attacking and just want a word other than "those brown people that blow themselves up."

2. Negative atheism is often used as a straw man for racism. A lot of neo-nazis are actually atheist and will post racist and hateful rhetoric that's disguised as "atheist criticism against Islam" so that it doesn't get taken down by mods or so the author can fall back on "its not racist because its a religion."

3. The vast majority of criticism against Islam ignores a ton of sociopolitical history and attribute acts and practices to the religion that are actually more regime-based than anything. Islam is America is VERY different from Islam in Iraq or Afghanistan. The religion is used as a tool for oppression in some regions, but that is no different than if any other religion were dominant. That is absolutely a problem for religon, but it not just an Islamic problem. Most muslim women in America that wear hijabs do so because they want to. American mosques are generally a more welcoming, generous, and peaceful place than many Christian churches. The majority of Muslims across the world actually describe themselves as passivists and condemn the acts of fundamentalists. Criticizing the sociopolitical practices of a few regions as a specific feature of Islam simultaneously insults muslims, ignores real historical factors, and ignores the true plight of a wide swath of ethnicities that - tbh, western countries absolutely contributed to. It's hard to know what exactly that is, a racism is at least one of them.

I have no idea if any of these things apply to the comment that got OP called racist. But if they do, I think getting called racist would be justified. That said, it's not for me to decide. It's for the responder to decide. OPs job is either to hand wave it and move on, or to find out why his comment was called racist and reevaluate whether his criticism is aimed at the right people.