Did Constantine invent Christianity?

No. Christianity had already existed for around 280 years by the time Constantine became emperor—it began in the first century, and Constantine ruled in the fourth.

What Constantine actually did: after a decisive military victory in 312, he moved to legalize Christianity across the Roman Empire the following year, ending large-scale persecution. He later called and presided over a major church council, but didn't personally invent its theology.

He didn't compile the Bible, and he didn't invent the idea that Jesus was divine—Christians were already worshiping Jesus that way for centuries before him, described even by a non-Christian Roman official writing around 112, decades before Constantine was born.

Constantine gave Christianity real political power and protection it hadn't had before, which changed its situation significantly. That's a genuinely important shift in Christian history. It's a different claim from saying he created Christianity, which the timeline alone rules out.