Oh dear.
Emperor Constantine united the Roman Empire under christian flags if you must call it that yes. He fought with christian symbols. True, true. So yes, at one stage the Roman Empire was Christian. I never stated otherwise.
Your problem however is that the Roman Empire was christian in the 4th century AC. Now Christianity was born in the 1st century AC. Between the 1st and the 4th lie 300 years. That's quite the gap, isn't it?
Now this is what you are suggesting: christianity, you say, was created as an excuse to hate the Jews. The Romans, you say, might have helped, because they weren't fond of Jews. So this is your hypothesis:
The Romans, who emphatically persecuted christians at the time, have helped christianity, which is only an excuse to hate Jews, to rise because both Romans and christians hated Jews.
Hm.