115:12-15 Trust leads to blessing for all people. The writer made this connection by repeating the same groups (cf. vv. 10-11a and 12-13a). In bestowing blessing God does not allow worldly greatness to influence Him. He is gracious to all. The psalmist wished God's blessing on all His people. Since He made heaven and earth, He is able to bless, and His blessing can be abundant.
115:16-18 The heavens are the Lord's domain, not that of pagan gods. He owns them, and He has given the earth to man for his habitation. It is important for God's people to praise Him while they live on the earth. After they die they cannot worship Him as they do now and so draw others to honor Him. Consequently the writer said he and the rest of the godly would bless the Lord forever. The final line calls everyone to praise Him.
The contrasts between the true God and idols are indeed great. God's people should review and appreciate these differences and so worship Yahweh for His unique individuality.
