Blessed is the nation whose God is the LORD; and the people whom he hath chosen for his own inheritance.
Second half of the main body (Psalms 33:12-19); the blessedness of the elect people whose God is the Lord; in Him they have a strength which multitudes of men and horses could not give.
Blessed is the nation - the central thought of the psalm. Israel primarily is the nation; then all the elect.
The people ... his own inheritance (Exodus 19:5; Deuteronomy 7:6; Amos 3:2) - spiritually the Church (1 Peter 2:9).