The gemara, brought in shu”a, says that when you hear half of a bracha made by a kusi, you don’t answer amen (we’re talking about brochos, shehakol, birkas hashavhar…) because he might have directed the bracha at Har Grizim. This is because it is known that some kusim believe in Hashem, but many are believers in shituf, as far as i know.
Evangelicals believe in the Trinity. There’s no doubt who they’re referring to when making blessings. But I’m not talking about blessings here – Christian speakers like to have their audience respond Amen to statements that they make which they want affirmation of. I believe the pastor said something he wanted a show of support for, but to use the word Amen to an idolator in a public mass gathering of Jews is a chilul Hashem. Having him speak at all is also a chilul Hashem.