Interesting... because Welcome Back, Kotter started airing in September 1975; and Travolta joined Scientology in... 1975.
Yes, seems to support some people's assertions that members of Scientology benefitted, in more than a spiritual way, from the association and were required to give back.
This religion has been around since the 50's, so JT probably dabbled for a while before committing. I'm sure he didn't have much choice after he won the role - if he wanted to keep it!
Who knows what was going on between him and the hierarchy of the church. He's made statements about not agreeing with them before.
We know how they work. 
But, I'm talking career-wise.)
) but I had to google a couple of terms in this thread.