Psalms 45:6

We have here the royal bridegroom filling his throne with judgment and keeping his court with splendour. I. He here fills his throne with judgment. It is God the Father that says to the Son here, Thy throne, O God! is forever and ever, as appears Heb 1:8, Heb 1:9, where this is quoted to prove that he is God and has a more excellent name than the angels. The Mediator is God, else he neither would have been able to do the Mediator's work nor fit to wear the Mediator's crown. Concerning his government observe, 1. The eternity of it; it is for ever and ever. It shall continue on earth throughout all the ages of time, in despite of all the opposition of the gates of hell; and in the blessed fruits and consequences of it it shall last as long as the days of heaven, and run parallel with the line of eternity itself. Perhaps even then the glory of the Redeemer, and the blessedness of the redeemed, shall be in a continual infinite progression; for it is promised that not only of his government, but of the increase of his government and peace, there shall be no end (Isa 9:7); even when the kingdom shall be delivered up to God even the Father (Co1 15:24) the throne of the Redeemer will continue. 2.

— Matthew Henry's Concise Commentary (public domain)

No lawful construction can be devised to change the sense here given and sustained by the ancient versions, and above all by Paul (Heb 1:8). Of the perpetuity of this government, compare Sa2 7:13; Psa 10:16; Psa 72:5; Psa 89:4; Psa 110:4; Isa 9:7.

— Jamieson-Fausset-Brown Commentary (public domain)