I think I'd be more willing to go that route as well. Something like: "Is the truth of Christianity depend on the literal historicity of the Torah?"
Although I think it could be framed better, something that would both take into account that your "Yes" answer isn't the answer of a literalist Christian but rather of a non-religious historian, and take into account the fact that my answer is not an absolute "No" but a qualified "No."
However it's worded, I think a good jumping off point would be the argument Paul makes in Romans 9-11, especially his thematic statement in 9:4-5: "They are Israelites, and to them belong the adoption, the glory, the covenants, the giving of the law, the worship, and the promises. To them belong the patriarchs, and from their race, according to the flesh, is the Christ, who is God over all, blessed forever. Amen."