Spell it out, Wonderland.
There was no God-model formulation handed down by the Apostles. In the earliest years of the primitive Church, emphasis was given to a biblical mode of living rather than developing theology. As the need for apologetics emerged to combat Gnosticism and Polytheism, various formulations came about. All were primarily to maintain Monotheism while accounting for Jesus.
With variations, six basic God-models became well-developed from the late-1st century onward. One of those was Sabellianism/Monarchianism/Patripassianism, which is (are) the original forerunner(s) for modern Oneness doctrine. It wasn't that the Father was the Son, so much as it was One God manifested in various ways, including as the Son.
Trinity "won" the contest of God-models, and has declared all others anathema and heretical. Unitarianism, Binitarianism, Tritheism, Arianism, and Sabellianism have all been anathematized by the orthodox church and all other trinitarians.
All ecclesio-political leverage aside, Oneness is as valid as Trinity. Modes and Persons both need to give way to truth. The most annointed men of God I've known have all been Oneness.