Its not ambiguous at all. Throughout history there has been an “inner need” for God, and for nearly all of humanity through all of history, nature has provided the evidence needed. Once you recognize it, and open yourself to God, God reveals more to you in your spirit. Its not just believing a historical message, My point was though, other things in history are accepted as iron clad absolute truth, while atheists dismiss the historical evidence of thousands of years of faith. History should be treated the same regardless of what it is. Its not honest to dismiss the evidence and say there isn't any.
Its only in the modern era, as we have gotten away from nature, that people have claimed there is no evidence for God. And there is plenty of evidence for the historical Jesus.
The message is also not limited to geographical boundaries. It has spread and there are probably only a very few, if any, people groups now who have not heard.
In the thousands of years before Christ, people believed in god. They still had a relationship with GOd, but were still waiting for the main event so to speak.
I am a little heretical in thinking it is not that big of a deal for those that have not heard. I think all religions are interpretations of the revelation that is in nature.. If someone has believed and honored the god they understand, maybe God will tell them his name … and they will say, oh, cool.