Your question is problematic, as you say the religion is invented, there is no religion called Judaism or Christianity. Judaism is derived from the name Judah, the son of Jacob. People named the religion of the son of Judah as Judaism. Christianity is Still an invented term, Jesus did not say I am Christian, this is also an invented term. There is only one religion called Islam which means submission to will of God according to which Adam was a muslim so were Noah, Moses, Jesus and many other prophets that lived in-between. We did not invent the term Islam, it means submission. If you had a chance to ask Jesus, what is your religion, he would not say I am Christian, because he did not heard it, also if you called him "Jesus, Jesus" he would not turn to you because he did not also hear the word Jesus, he was Hebrew not a Grek, the name Jesus is a Grek word for as far as I remember "Isoiah" or something sounding similar, and in Arabic "Isa". Lets come to your question, Allah (God) says in the Quran that He has sent many messengers except those named in the Book, therefore all the names of prophets are not recorded. As an Islamic rule, those who were not delivered the message are not to be held responsible, therefore anyone who did not hear the message of Lord will not be responsible. But today, this is not the case, media, universal communication spreads information instantly. Allah will deal with past generations fairly as he says (3/182) That is for what your hands have put forth and because Allah is not ever unjust to [His] servants."
Leave past generations to themselves and care for yourself, we wont be asked what elders did but what we have done.