13 “For requiring their blood He has remembered”. As if they, who were sent to preach the Gospel, should make answer to that injunction which has been mentioned, “Show forth His wonders among the heathen,” and should say, “O Lord, who has believed our report?” and again, “For Your sake we are killed all the day long;” the Psalmist suitably goes on to say, That Christians not without great reward of eternity will die in persecution, “for requiring their blood He has remembered.”
But why did he choose to say, “their blood”? Was it, as if one of imperfect knowledge and less faith should ask, How will they “show them forth,” seeing that the infidelity of the heathen will rage against them; and he should be answered, “For requiring their blood He has remembered,” that is, the last judgment will come, in which both the glory of the slain and the punishment of the slayers shall be made manifest? But let no one suppose “He has remembered” to be so used, as though forgetfulness can attach to God; but since the judgment will be after a long interval, it is used in accordance with the feeling of weak men, who think God has forgotten, because He does not act so speedily as they wish.
To such is said what follows also, “He has not forgotten the cry of the poor:” that is, He has not, as you suppose, forgotten. As if they should on hearing, “He has remembered,” say, Then He had forgotten; No, “He has not forgotten,” says the Psalmist, “the cry of the poor.”
Source: The Enarrations, or Expositions, on the Psalms (New Advent)