"The Trinity was invented at Nicaea by Constantine in 325"

The Trinity is a fourth-century invention. Constantine forced it through at the Council of Nicaea in 325 for political reasons. Before that, Christians did not believe Jesus was God.

Nicaea was called to settle a dispute that a priest named Arius had started by teaching that the Son was a created being. The council's answer — that the Son is "of one being with the Father" — was a defence of what the churches already confessed, not a novelty. Constantine convened and presided for order; he was not a theologian, he was not baptised until he was dying, and he later leaned toward the Arian side. The idea that he authored the doctrine does not survive contact with what he actually did.

Even if Constantine didn't write it, the doctrine still wasn't there before the fourth century.

It was there in substance. Paul, writing in the 50s, applies the Shema itself to Jesus — "one God, the Father… and one Lord, Jesus Christ" — and closes letters with a three-part blessing of Father, Son, and Spirit. Matthew has baptism "in the name of the Father and of the Son and of the Holy Spirit." A generation later, Ignatius of Antioch (about AD 110) simply calls Jesus "our God" in passing. Justin Martyr, Irenaeus, Tertullian, and Origen all teach that the Son and the Spirit are truly divine and personally distinct from the Father. Tertullian is using the word "Trinity" around AD 200.

But those early writers didn't agree on the details. The doctrine developed over time.

The vocabulary developed — words like "of one being," "person," "procession" were sharpened over two centuries of argument. That is normal: the church had to find precise language to say what it had always done, which is worship one God as Father, Son, and Spirit. What did not develop is the core: from the earliest sources we can reach, Christians pray to Jesus, are baptised into the triune name, and refuse to call him a creature. Nicaea is a fence built around that, not the thing itself.