The magnificence of God demands that the study of Him requires more than our finite minds can offer. This is certainly true when it comes to studying God as Father, Son and Spirit; God as Trinity. In the first centuries the doctrine was not defined but it was still, simply held. From the beginning the disciples recognized God in Three Persons, One Essence.
John Owen, the Puritan wrote, “the sum of this revelation in this matter is, that God is one ; — that this one God is Father, Son, and Holy Ghost ; — that the Father is the Father of the Son ; and the Son, the Son of the Father ; and the Holy Ghost, the Spirit of the Father and the Son ; and that, in respect of this their mutual relation, they are distinct from each other.” 1
The apostles heard Jesus order them to baptize in the (singular) name of the Father, the Son and the Holy Spirit. They heard Jesus proclaim that the Spirit would be another of the same kind. They recognized and rightly worshiped Jesus as fully God. And they heard Jesus proclaim that He and the Father were one, and demonstrate it by His works. Hearing and seeing and experiencing God in the flesh before them as they did, they were led inexorably to the unity of God in trinity.
The Trinity is one of the most boggling concepts in the scripture, and all of our struggles to comprehend it fall short.
One of the most complete statements on the trinity is the Athanasian creed. Written perhaps around 500 AD (up to two hundred years after Athanasius (died Ca 373AD))