"Peace and unwavering trust in God pervades this psalm [120/121]. It puts to rest all disquietude in the soul and even in our days inspires us to peacefulness; for what the psalmist says of his sanctuary on Sion and of the throne of God in the Holy of Holies may be said in a greater measure about the tabernacle of the Catholic house of God. The psalmist begins with a confession of confidence (verses 1-2). The Israelites when far away from the sanctuary were accustomed to pray facing Jerusalem on the Judean mountains, the Mount Sion on which the temple was built. So also the psalmist directs his view to the holy heights that greet him from the distance. There the Lord lives among his people; there his eyes are always watching the affairs and needs of those who pray to him; there his ears pay heed to their petitions. There the God-fearing man in faith and confidence may expect help in every visitation; for the God who broods there over the Ark of the Covenant is Yahwe, the invisible King and Father of Israel who has obligated himself to his people by a solemn covenant; it is the kind and eternal God, the almighty Creator of heaven and earth, for whom there are no impossibilities. If he has created the universe with one word, then he knows also how to banish the needs of men with one word.
From the house of God or from his own deeply pious soul the psalmist hears the divine Amen to his confession of confidence (verses 3-7). The Lord will not let him stagger and fall who in the confidence of his love takes hold of his hand. God supports everyone who in humility does not rely on his own strength alone, but in the consciousness of his weakness seeks support from God. The good Lord continually watches over such a one. God's eyes do not tire; he does not fall asleep; he does not overlook the stumbling petitioner. It is unthinkable that Yahwe who has taken upon himself the protection of Israel will forget and be unmindful of what he voluntarily undertook. It is therefore out of the question that the enemy will sow destruction while the Lord sleeps. Whatever happens under the watchful eyes of God happens with his will and permission. The thought that God does not bother about the sufferings of the just should never enter one's mind. Indeed, God protects his children at all times and in everything that occurs. He is not far away but stands protectingly at our right. He is mightier than the guardian angel whom he has told to carry the just in their hands that they may not stumble upon a stone. He guards them against all evil by day and night, from the heat of the sun during the day and from lunacy at night. In the Orient those who became ill because of the chill evening temperature were said to become sick through the influence of the moon. There is no suffering from which the Lord cannot keep us; there is no danger from which he cannot save us. Under his protection is the going and coming of man; that is, man's doings and omissions are under God's powerful protection now, in the future, and throughout an entire life."
-- Herder's commentary on the psalms edited by Edmund Kalt