Sourced guide
Was the Trinity invented at Nicaea?
No. Nicaea did not invent the Trinity. It gave conciliar boundaries and vocabulary to a dispute about the Son's divinity that was already visible in earlier Christian writers.
What Nicaea did
The Council of Nicaea in AD 325 answered the Arian controversy by confessing the Son as of the same substance as the Father.
That was a formal council definition, not the first time Christians worshiped Christ or spoke of Father, Son, and Spirit.
The pre-Nicene trail
Ignatius, Justin, Irenaeus, Theophilus, Tertullian, and Origen all sit before Nicaea in the data. They do not use one identical later formula, but they show the question was already inside Christian theology.
The post-Nicene figures, especially Athanasius and the Cappadocians, clarify and defend the Nicene settlement rather than starting from nothing.
Relevant relationships
Irenaeus quotes Justin's lost work Against Marcion by name and follows Justin's anti-heretical scheme.
Basil and Gregory of Nazianzus compiled the Philocalia, an anthology of Origen.
Co-compiler with Basil of the Philocalia of Origen.
Surviving correspondence and the joint Philocalia of Origen.
Basil wrote to Athanasius seeking unity with Rome.
Basil and Gregory were close friends from their student days at Athens.
Gregory describes Basil as his teacher and elder brother.