Using Muslims as an example they reject a strawman argument about Christ and when they hear Christ they see the strawman, "There is no other God but God", "God cannot have children", "God does not have sex" etc...; but what they reject is something we also reject. The Quran is vehemently anti-tritheistic and monotheistic it is also aggressively Unitarian, but intentionally seems to avoid anti-trinitarian language. They reject a false construct of Christ but in terms of the true Christ, they do know him.
It's like in a political campaign there is a lot of misinformation and propaganda, one may reject a candidate based on this misinformation but what are they really rejecting? the candidate or the misinformation? Perhaps the same person meets the candidate who is able to inform them of the truth, now that they know the truth they may continue to reject or not but the rejection now has more weight.
Of these religions that can be argued as the same referent of the true God do they know Christ? Do they even know God? What did Paul say in Acts 17 "what therefore you worship as unknown..." They may reject a heretical version of Christ but is this really rejecting Christ himself? How can they reject something they do not understand or know? I'm speaking more broadly and there are those who have been introduced to the true Christ yet still reject him, this, of course, is more conclusive but as a whole many other faiths reject misinformation about Christianity, or perhaps reject culture or offences but often don't have the opportunity to reject the true Christ.