The phrase "strong believer" refers to someone who has a deep conviction or faith in something. It means that they have a strong and unwavering belief in a particular idea, concept, or cause. Full definition
I am a VERY strong believer in the separation of church and state (and I mean VERY), yet I do not see how this should be a problem.! (religion.blogs.cnn.com)
Now, I will admit that I haven't always been as strong a believer in the importance of taking time off as I am today. (inc.com)