Altruism is simply a concern for others. In broad sense it can mean that you care whether others get more value (according to either receivers' opinion or your opinion about values).

"Effective" just means you don't want to waste money or time on values not important (according to either receivers' opinion or your opinion).

The fact that Givewell chooses certain values (the rational ones) and Christianity slightly different values is orthogonal to the concept of altruism.

Even if I believed in Flying Spaghetti Monster I could care whether others have a steady supply of macaroni. This makes me an altruist in my book. And I could act effectively about it. But I wouldn't complain about Givewell in that case.

And that's why I don't like conflating rationalism with effective altruism. It's just another case of emotional loading of a phrase.

https://en.wikipedia.org/wiki/Flying_Spaghetti_Monster