OP the religion of Christianity arose as a result of the destruction of the Jewish temple and the Jewish priesthood by the Romans in the year 70. This date marks the inception of the religion and not AD30-33 when Jesus was supposedly crucified.
Given that the two religions, CHristianity and Judaism, continue to exist, the idea that Christianity was somehow invented as an alternative to the Temple destruction is, at best, an odd one.
If one reads the Book of Acts, we find numerous references to Christianity (and to Judaism) as existing.
Acts 2 Pentecost, pouring out of Spirit
Peter's sermon, mentioning numerous fulfilled prophecies
Acts 6, choosing of seven
And the word of God increased; and the number of the disciples multiplied in Jerusalem greatly; and a great company of the [Levitical] priests were obedient to the faith.
Acts 8, Christianity spreads out from Jerusalem and Alexandria
Acts 10, Christianity spreads to Gentiles
Acts 11, the name Christians is first used, used to differentiate from Judaism
Acts 15, conflict between the two religions on "how to be saved"
Acts 16-28, spreading of Christianity
OP it is asserted that the life of Jesus as portrayed in the gospels was invented to explain how the religion arose, but it actually arose as a consequence of the interplay of expectation and traumatic events consistent with normal human behaviour.
Per Daniel's 70 weeks, the expectation of Messiah was high and fulfilled within its time frame. This was 40 years (symbolic of testing) prior to Temple destruction.
So, to answer the OP, Christianity existed from Christ's resurrection and ascension and Pentecost through today. The destruction of the Temple was foretold by Christ, which meaning is a different question.