17 The Apostle here speaks in carefully guarded words, which by their definiteness can give no occasion to the ungodly. We have seen that the Evangelist makes the Lord use the word 'Brethren' in the preface to the message, thus signifying that the whole message, being addressed to His brethren, refers to His fellowship in that nature which makes Him their brother. Thus he makes manifest that the mystery of godliness, which is here proclaimed, is no degradation of His divinity.
The community with Him, by which God is our Father and His, our God and His, exists in regard to the Dispensation of the flesh: we are counted His brethren, because He was born into the body. No one disputes that God the Father is also the God of our Lord Jesus Christ, but this reverent confession offers no occasion for irreverence. God is His God but not as possessing a different order of divinity from His. He was begotten God of the Father, and born a servant by the Dispensation: and so God is His Father because He is God of God, and God is His God, because He is flesh of the Virgin.
All this the Apostle confirms in one short and decisive sentence, Making mention of you in my prayers that the God of our Lord Jesus Christ, the Father of glory, may give unto you a spirit of wisdom and revelation. When he speaks of Him as Jesus Christ, he mentions His God: when his theme is the glory of Christ, he calls God His Father. To Christ, as having glory, God is Father: to Christ, as being Jesus, God is God. For the angel, when speaking of Christ the Lord, Who should be born of Mary, calls Him by the name 'Jesus:' but to the prophets Christ the Lord is 'Spirit. ' The Apostle's words in this passage seem to many, on account of the Latin, somewhat obscure, for Latin has no articles, which the beautiful and logical usage of Greek employs.
The Greek runs, ὁ Θεὸς τοῦ Κυρίου ἡμῶν ᾿Ιησοῦ Χριστοῦ, ὁ πατὴρ τῆς δόξης, which we might translate into Latin, if the usage of the article were permitted, 'Ille Deus illius Domini nostri Jesu Christi, ille pater illius claritatis' (The God of the Lord [of us] Jesus Christ, the Father of the glory). In this form ' The God of the Jesus Christ,' and ' the Father of the glory,' the sentence expresses, so far as we can comprehend them, certain truths of His nature. Where the glory of Christ is concerned, God is His Father; where Christ is Jesus, there the Father is His God. In the Dispensation by which He is a servant, He has as God Him Whom, in the glory by which He is God, He has as Father.
Source: On the Trinity (New Advent)