To get back to the original question.
How many terrorists there are really depends on what is a terrorist. Asking how many terrorists are in the world, is like asking how many idiots are in the world or how many beautiful people are in the world. It all depends on your definition.
To respond to Ayn Ryan example of people who believe in Shira Law are terrorists. If i take this on face value this is a narrow definition. It also raises some interesting questions. Are they still terrorists if they only want Shira Law if it is voted in directly or indirectly in a democratic country (doesn't make it moral)? Are the respondents willing to use the direct use of force to impose Shira law? Is Shira Law the only belief that makes you a terrorist? Would other beliefs also make you a terrorist? To treat Ayn Ryan as a rational human being and stop straw manning him/her. Does the belief in something make you a terrorist? If so what/who determines if that belief makes you a terrorist?
A quick google search, provides many vague definitions of varying quality. As an example, please see the FBI's definition of international terrorism below:
Source: https://www.fbi.gov/about-us/investigate/terrorism/terrorism-definition
That narrows it down a bit. So if i got in my mind that there should be a law that everyone has to wear purple hats and i go and do something violent i would be a terrorist. Neat!
Where it get's fuzzy, is what does "involve" mean? Does the act have to carried out? or can it be the planning phase? Do i have to pull the trigger or am i a terrorist if i get the bomber's coffee? Also it's totally OK to commit violent acts and acts dangerous to human life if it's not breaking the law. Just an amusing side note.
The second criteria, could literally mean anything. For example politicians fit into "to intimidate or coerce the civilian population", it's kind of their job.
The third criteria is just to separate "Domestic" and "international terrorism". So i won't address it.
Interestingly, when you combine that definition some really odd things could be called terrorism.
If i kidnapped and then murdered someone in Romania, for any reason by that definition i would be a terrorist. I committed violent acts (A) which would intimidate the people in Romania (B) and last i checked Romania is not in the US ©. That last part might need a fact check.
I think terrorism is one of those "trigger words". It has a lot of emotional impact behind it. It's a real gut punch, kinda of like rape but without the concise definition. So i don't think there is a way to say how many terrorists are in the world because the definition to terrorism is too board and people can't agree on what it is. I don't think the FBI is or should be the definitive source of the definition. If you have a definition in mind, i can drill into that and hopefully give a better estimate. This is a really interesting question to think about. I don't think Islam and Christianity has much to do with it though. These religions are part of the current terrorism problems, but your question is more abstract then that. It's not like Islam has a patent on terrorism. There has been many terrorists, from many different belief systems.