Olog-hai:
Christ is the first-born of many brethren relates to his Human nature, he is the first-born of a new humanity that has been saved by God’s grace, just as the first man Adam caused the downfall of humanity, the second-Adam is the “perfect Man”, i.e. the Divine Person Jesus Christ as a perfect human nature and thus all of us baptized into the Trinity are now the adopted sons and daughters of God adn thus brothers of CHrist. All of us who are baptized into the Trinity and believe in the Holy Trinity and all that has been revealed by Christ are born anew via baptisim which cleanses original sin and all sin, makes us a member of God’s Family, thus brothers and sisters of Chrsit and members of Christ Body the Church.
In summary, the goal of God’s plan of salvation (cf Roman 8: 29-30) is aimed at conforming all who come to know the One True God and believe in him into the image of Christ and thus it is aimed at the final glory of all who put their faith in Christ.
The “triune” trinity is non-Biblical. In fact Jesus prayed that his desciples would be one even and he and his Father are one.
“And the glory which thou gavest me I have given them; that they may be one, even as we are one:” (John 17:22)
It’s clear that there are three personages, but Eusibius said that it was Constantine (a pagan) who came up the “triune” notion.