Title text: I lead a small but extraordinarily persuasive religion whose only members are door-to-door proselytizers from other faiths.
Using percentage of growth can be a misleading value to use for gauging the importance or popularity of something. If you have 5 members and then add 5 more, that would mean you have achieved a growth of 100%.
In the case portrayed in this comic the claim appears to be that the other person's religion grew by 85%. Black Hat attempts humorously to show the flaw in using that statistic by growing his group by 100% (therefore, presumably, first place), which he simply does by adding his friend Rob to his religion, and thus increasing his membership from 1 to 2. The other person then says that his religion has a significant number of members (and not just one or two), but Black Hat doesn't care and responds that he hopes they are all OK with being "in second place" since the main argument from the other guy was about being the fastest-growing.
The title text is probably Black Hat parodying the entire point of having a religion. He managed to snag himself a door-to-door proselytizer who tries to convert people to a faith that is not Black Hat's faith of door-to-door proselytizers from other religions. Black Hat himself is probably a door-to-door proselytizer from yet another faith.
Another interpretation is that the title text could be another way that Black Hat could take the 'fastest-growing' claim out of context to make it meaningless. By composing his religion of the unwitting proselytizers of other faiths, he can claim the highest ratio of converts to current adherents. Note that the amount of people converted is often exaggerated by groups that try to spread a faith. Although the beliefs spread by his proselytizers vary widely, Black Hat is not concerned with what his so-called followers believe. Thus, he can claim the title of fastest-growing religion without having any value to his religion.
- [A man with a combover, a book, and a clipboard approaches Black Hat.]
- Combover: You should check us out. We're the fastest-growing religion in the country.
- Black Hat: "Fastest-growing" is such a dubious claim.
- Combover: It's true! We grew by 85% over the past year.
- [Black Hat shouts to someone out of frame.]
- Black Hat: Hey, Rob — wanna join my religion?
- Rob: Sure, whatever.
- [Black Hat turns back to Combover and produces a notepad and pen.]
- Black Hat: Well, looks like my religion grew by 100% this year.
- [Black Hat begins to walk away.]
- Combover: We have 38,000 members!
- Black Hat: Hope they're all ok with second place.
- There is a wiki page on the subject here: "Growth of religion".