Psalms 89:52

Blessed be the Lord for evermore
In this world, and in the world to come, as the Targum; for reproaches and afflictions for Christ's sake, since they work together for good; as Job blessed the Lord in the midst of his troubles, ( Job 1:21 ) , or rather the psalmist, viewing, by a spirit of prophecy, Christ rising from the dead, ascending to heaven, sitting at the right hand of God, and interceding for the application of all the blessings of the covenant; and now, seeing all before objected and complained of was reconcilable to the love, covenant, and oath of God, breaks out into this benediction, and with it closes the psalm; which agrees with Christ, not only as God over all, blessed for ever, but as Mediator, who, as such, is made most blessed for evermore; see ( Psalms 21:6 ) . These are not the words of the copier of the Psalms, blessing God for assistance in prosecuting the work thus far, which is the sense of some Jewish writers mentioned by Aben Ezra and Kimchi, but of the psalmist himself:

Amen, and Amen;
which words are added to express the wish and faith of the psalmist; and the word is repeated to denote the vehemence and strength of the same. Here ends the third part of the book of Psalms, and so the Syriac version closes it. (See Gill on Psalms 41:13). (See Gill on Psalms 72:20).