15 These, then, are the words with which He prefaces the message, Go unto My brethren, and say to them, I ascend unto My Father and your Father, and My God and your God. I ask, Are they to be understood as His brethren with reference to the form of God or to the form of a servant? And has our flesh kinship with Him in regard to the fullness of the Godhead dwelling in Him, that we should be reckoned His brothers in respect of His divinity? No, for the Spirit of prophecy recognises clearly in what respect we are the brethren of the Only-begotten God.
It is as a worm and no man that He says, I will declare Your name unto My brethren. As a worm, which is born without the ordinary process of conception, or else comes up into the world, already living, from the depths of the earth, He speaks here in manifestation of the fact that He had assumed flesh and also brought it up, living, from Hades. Throughout the Psalm He is foretelling by the Spirit of prophecy the mysteries of His Passion: it is therefore in respect of the Dispensation, in which He suffered, that He has brethren.
The Apostle also recognises the mystery of this brotherhood, for he calls Him not only the firstborn from the dead, but also the firstborn among many brethren. Christ is the Firstborn among many brethren in the same sense in which He is Firstborn from the dead: and as the mystery of death concerns His body, so the mystery of brotherhood also refers to His flesh. Thus God has brethren according to His flesh, for the Word became flesh and dwelt among us: but the Only-begotten Son, unique as the Only-begotten, has no brethren.
Source: On the Trinity (New Advent)