This psalm “makes a proper distinction between the fate of righteous and the fate of the wicked, a favorite theme in Israel’s wisdom poetry (cf. Ps. 1; Prov. 1-9). Those blessed are those who trust in the LORD (vs. 1), the righteous (vs. 3), the good (vs. 4a), those who are upright in heart (vs. 4b). Wicked kings (vs. 3a) and people (vs. 5) must be eliminated so that the righteous will not be encouraged to do wrong (vs. 4)” Miller, 404.

125:1 Are as Mount Zion, which cannot be moved but abides forever- Ps.68:15-16 The verb moved is translated shaken in Ps. 13:4; 15:5; 16:8; 21:7; 55:22; 62:2, 6; 82:5; 112:6 or slips in Ps. 17:5; 38:16; 46:2; 94:18 or moved in Ps. 46:5; 93:1; 96:10. “The same word is translated in Ps. 121:3 as stumble”- NICOT, 911. “Mountains are often symbolic of stability and endurance, particularly in contrast to the waters of chaos (Ps. 46). And here, the psalmist names the most important mountain of all, Zion, the location of the temple” Longman, 422. “He draws upon the traditional picture of impregnable Zion (cf. Ps. 46; 48; 76; Isa. 28:16)” Laymen’s, 689

125:2 As the mountains surround Jerusalem- “The second (simile) compares the LORD Himself and the mountains that surround Jerusalem” Broyles, 455. “Mount Zion is not the highest peak in the mountain range around Jerusalem. To the east lies the Mount of Olives, to its north Mount Scopas, to the west and the south are other hills, all of which are higher than Mount Zion” VanGemeren, 788. The same word for surrounds is used in both parts of the verse. “The mountains offered security to Jerusalem and became a symbol of divine protection” Miller, 404. 

So the LORD surrounds His people- “God is ‘around’ and present with His people (Ps. 34:7; Zech. 2:1-5)” VanGemeren, 788.
 
125:3 “Prophets constantly rebuked Israelite leaders for adversely influencing the nation through their own wickedness (cf Is. 1:21-31; Jer. 5:1-6; 6:13-15; Hos. 4:4-10). The psalmists acknowledged the principle also (cf. 72:1-4; 78:67-72; 101:1-8)” Miller, 404. Neh. 9:36-37 depicts foreign rule over Jerusalem but may describe the situation the people were in. The word for scepter can mean rod and “the sense of punishing force may be the more relevant one here” Alter, 446.

125:4 Do good, O LORD, to those who are good- Ps. 18:20-27 This petition is the only place in the psalm where God is directly addressed. “It seems deliberately nonspecific and so leaves the particular means to Yahweh’s discretion” Broyles, 456. This also contains a warning to the congregation listening that they must do good. 

125:5 But as for those who turn aside to their crooked ways- This contrasts with those who will not be moved in vs. 1. “Nationality or ethnicity does not guarantee Yahweh’s favor-one must maintain a relationship to Yahweh and a moral heart” Broyles, 456. The ‘evildoers’ are apostates who have turned to ‘crooked ways’ (vs. 5), i.e. paths that twist and turn away from the main road (cf. Judges 5:6)” VanGemeren, 789.

Peace be upon Israel- Ps. 120:6,7; 122:6-8; 125:5; 128:6- “Israel is equated with those who trust, the righteous, the good and the upright” Laymen’s, 689. Numbers 6:26; Israel stands for all God’s faithful people- Galatians 6:16; Rom. 2:29; John 1:47.