The first Christians were Israeli Jews in the 1st century AD. They put their faith in Jesus as the Messiah, the savior sent from G-d, the one whom the prophets had foretold would bring about redemption for Israel and all mankind. Their written testimonies of Jesus life and teachings are collectively called the New Testament or more precisely the New Covenant Scriptures (the name being a reference to the new covenant that Jesus ushered in, the covenant that was promised by G-d through the prophets). The New Covenant scriptures, together with the Tanach (the Hebrew Bible), are the true oracles of G-d. The new is in the old concealed and the old is in the new revealed. In them and them alone is found truth, life and salvation. They are G-d's message of salvation for all mankind, given to mankind through the Jewish nation.
The testimony of his first followers, those 1st century Israeli Jews who wrote the New Testament, stands the test of time. His own words stand every test for they are the Truth. Read it for yourself...