These are old articles, but a paper on World Englishes that my friend Caroline delivered at HSU's Fall English Studies Conference got me interested in doing a little more reading on the subject. She mentioned Christian ESL teachers whose not-so-implicit goal is, essentially, to "destroy Islam" (that's how the Mother Jones article describes it, anyway).
Anyway, that led me to a very thoughtful article in Christianity Today by a Nonnative English speaker who learned English -- and converted to Christianity -- due to the influence of a Christian English teacher.
As for whether Christian ESL teachers should evangelize, if you know me I think you won't be surprised that I don't believe they should -- or at least that they should be honest to everyone about what their "true" goals are. I agree with Seaman (whose work on cross-cultural perception I used to write a paper about comics in ESL Writing instruction), who says in the CT article "You don't go in as a prophet; you go in as a servant."
As Sayers wrote, "the only Christian work is good work, done well." To be a Christian ESL teacher is to simply and faithfully equip learners with the linguistic tools they need to communicate in English.
There are a lo of people in TESOL who are Christians -- Scovel, Seaman, and Canagarajah are a few high-profile names -- and I hope that aspiring teachers (like me) will note their humble approaches to teaching.