This question discusses both the obligation to give charity and certain requirements of the receiving institution, but the "best answer" does not address one particular type of charity. It mentions that a Catholic charity has "helped Jewish parents adopt a baby, too" but I don't know if this is meant to limit donations to that one particular, unnamed charity or to say that all Catholic charities are acceptable.
As I walked out of the store today, there was a table manned by representatives of the "Mission Life" church. That might not actually be the name but it was something like that. Their sign said that donations would help save a child's life but I don't know the scope of that -- are they saving his soul through Christian education? Are they providing actual medical help? But if I don't donate, would that appear as a chilul hashem as I am ignoring people in need?
What about donating my clothes to the Salvation Army -- their name makes clear one of the aspects of their charity, "Salvation." And if Santa is standing outside the store in mid-December, am I allowed to donate even though he is collecting money which might be used for something akin to missionizing?
In short: Is it at all acceptable to donate to a Christian charity if I don't know whether or not the money will go towards missionizing and the like?