Albert Barnes Commentary Psalms 122:8

"For my brethren and companions` sakes, I will now say, Peace be within thee." — Psalms 122:8 (ASV)

For my brethren and companions’ sakes - Because they live there; or, because they go up there to worship; or, because they love you, and find their happiness in you; or, because they are unconverted, and all my hope of their salvation is to be derived from you—from the church, from the influence of religion.

I will now say, Peace be within thee - I will pray for your peace, for your prosperity, for the blessing of God upon you—because their good, their comfort, their hope of salvation, depends on you—on the influence which will go out from you.

So the Christian prays that the church may prosper, that the divine blessing may rest upon it, that there may be in it harmony, peace, love, and zeal, and that a blessing may attend the preaching of the gospel—not only because he loves it and seeks his own comfort and edification in it, but also so that his friends and relatives—his wife, his parents, his children, his neighbors—those whom he loves, and whose salvation he desires, may be saved.

This expresses the true feelings of piety all over the world. This is one of the grounds of the strong love which the friends of God have for the church—because they hope and desire that through the church, those most dear to their hearts will find salvation.