St Gregory of Nyssa says concerning Christ being "first born among many brethren,"
"He becomes “the first-born among many brethren,” Who is born before us by the new birth of regeneration in water, for the travail whereof the hovering of the Dove was the midwife, whereby He makes those who share with Him in the like birth to be His own brethren, and becomes the first-born of those who after Him are born of water and of the Spirit [Jn 3:5].
In what sense then does He become “the first-born among many brethren?” ... He became the first-born of those who are spiritually born again, and gave the name of brethren to those who partook in a birth like to His own by water and the Spirit".