Spe Salvi

and how many Sundays has it been sung, "Let us build, the city of God"
hence it is helpful to read a reasonable voice; let us sing the old song in full harmony:

Certainly we cannot “build” the Kingdom of God by our own efforts—what we build will always be the kingdom of man with all the limitations proper to our human nature. Spe Salvi, 35.

what is our pope saying here. I found a clue in an old post from Fr. Z

His Eminence Josef Card. Ratzinger, in his wonderful book A New Song For The Lord: Faith in Christ and Liturgy Today (Crossroad Publishing Company, 1996) presents a reflection on the imagery of “living stones” which, though applied in his book mainly to seminary formation and priesthood, nevertheless is applicable to every Catholic in every walk of life, particularly today. His Eminence writes:

The goal is the house; what precedes it are the stones – living stones in the case of a living house. The fact that our verse talks about building in the passive voice is part of this: Like living stones, let yourselves be built into a spiritual house. Our thirst for action requires that we translate such words without exception into the active voice: Let us build the kingdom of God, the Church, new society, and so forth. The New Testament sees our role differently. The construction manager is God or the Holy Spirit. We are the stones – for us building means being built. An old liturgical hymn for the construction of a church describes this graphically; it speaks of the blows of the curative chisel, the thorough treatment with the master’s hammer, and the right assembly of the pieces through which the blocks of stone finally grow together into the great building of Jerusalem. This touches on something very important: building means to be built. If we want to become a house, we – each and every one of us – must accept the fate of being cut and carved. (pp. 163-164)

So to participate in the building up of the body of Christ, one must be prepared for the hammer and chisel; not to land the blows on the body, but to be the recipient from the Master Carver.

The atheism of the nineteenth and twentieth centuries is—in its origins and aims—a type of moralism: a protest against the injustices of the world and of world history. A world marked by so much injustice, innocent suffering, and cynicism of power cannot be the work of a good God. A God with responsibility for such a world would not be a just God, much less a good God. It is for the sake of morality that this God has to be contested. Since there is no God to create justice, it seems man himself is now called to establish justice. If in the face of this world's suffering, protest against God is understandable, the claim that humanity can and must do what no God actually does or is able to do is both presumptuous and intrinsically false. It is no accident that this idea has led to the greatest forms of cruelty and violations of justice; rather, it is grounded in the intrinsic falsity of the claim. A world which has to create its own justice is a world without hope. No one and nothing can answer for centuries of suffering. No one and nothing can guarantee that the cynicism of power—whatever beguiling ideological mask it adopts—will cease to dominate the world.
spe Salvi, #42.