I don't really understand the objection here. Nobody actually thinks Christ to have come only for the salvation of anatomically male people, and grammatical gender does not have to match with anatomical gender, even when we are talking about human beings, which are the 'most gendered' of all referents, being highest on the animacy hierarchy (see regarding this the works of linguist William Croft). In Spanish, for instance, if you are going to talk about martyrs, you are going to use a grammatically masculine noun regardless of the anatomical sex of the referent, martir. In that case, it is the accompanying pronoun which wil vary (el martir, la martir). In all my life as a speaker of this language, I have never heard anyone, male or female, point to this as an example of discriminatory or exclusionary language. Nobody ever says "What about women?!", because you know if you're talking about women by virtue of the fact that you're referring to women/a woman. It doesn't need to be expressed explicitly via the form of the noun, even in a language like Spanish which usually does things this way.
How much more, then, should we expect these kinds of 'mismatches' in a language like English, which generally does not mark gender via nouns? Yes, in the case of this particular word there is a distinction which can be made, but it is much more reasonable to argue that the distinction to be made in the context of the Creed in particular is one of collective vs. singular, rather than male vs. female, as (as Rusmeister has pointed out) nobody actually believes that Jesus Christ came only for the salvation of anatomically male people.
So if you are going to argue that the version in use does not include women by virtue of its form, that's essentially arguing that the collective noun must be explicitly marked (grammatically) feminine so as to not leave them out, when we have already seen from the above Spanish example that this is not how things work even in a language that expresses gender in its nouns much more regularly and transparently than modern English does.
As both a Christian and a linguist, that does not make a lot of sense to me. I'd like to hear some argument for it that is rooted in the way that the language actually works at a functional level, rather than how it makes us feel.