Because the early apologists such as Clement and Justin Martyr and Irenaeus all claimed in their apologies that the Greek god was the same as the Christian God and all they needed was to add Christ. To the contrary, even Tertullian understood the utter heresy of this position when he exclaimed most famously, "What has Jerusalem to do with Athens"?Tertullian doesn't define Christianity. Neither does the tribal god of the Jews. You have misrepresented the case. The early church acknowledged that all people had some knowledge of God, but incompletely. The choice between Athens and Jerusalem is a false one. In Christ there is neither Greek nor Jew.