You ask some good questions and make a strong case.
Indeed, the test of a god's existence used to be the verification of the god's power to act in this world here and now. Christians would point to the success of the church, which in line with what you say was largely a watering down of Gnostic philosophy and radical Judaism, a cooptation of a spiritual message by the Roman Empire for political purposes.
Other than than, Christians would point mainly to the miracle of their god's death on earth. That inversion is supposed to be profound, but it could also be interpreted as anticlimactic.
The book, "Creating Christ: How Roman Emperors Invented Christianity" takes this line of argument further (perhaps too far).