The main line of the Bible is the central line of God's economy concerning Christ and the church. This is the line of life. Christ is the centrality and universality of God's eternal economy. The church is the Body of Christ (Eph. 1:22-23), the counterpart of Christ, the complement of Christ, to be God's dwelling on the earth, typified by the tabernacle (Exo. 25:8-9; Josh. 18:1; 1 Sam. 1:3) and the temple (1 Kings 6:1), both of which are considered as the house of God (Judg. 18:31b; Psa. 5:7). The main line is of Mount Zion in the heavens (Psa. 2:6; Rev. 14:1), signified by Mount Zion on the earth (Psa. 3:4; 9:11, 14; 14:7; 15:1b); and the New Testament believers have come to Mount Zion in the heavens (Heb. 12:22) in the main line of God's economy.
The other line in the Scriptures is the side line concerning the law. This is the line of the knowledge of good and evil. The law was added alongside the main line of the economy of God (Rom. 5:20a). The side line is of Mount Sinai, where the law was given. The Old Testament saints, who were for the law, were slaves of this Mount Sinai (Gal. 4:24).
The psalmists, who were for the law, stayed ignorantly at Mount Sinai of this side line. Occasionally and unconsciously they turned to Mount Zion of the main line, as unveiled in Psalms 3:4; 9:11, 14; 14:7; and 15:1b. In Mount Zion they enjoyed the house, the temple of God, as unveiled in Psalms 5:7 and 11:4. This is an unconscious progression in the psalmists' seeking after God.
The psalmist in Psalm 15 was questioning as to who may sojourn in God's tent and who may dwell on God's holy mountain. The answer according to David's concept is the perfect man in the side line of the law (Psa. 15:2-5). But the answer according to the divine revelation is the God-man Christ (Psa. 16:1-11) as the firstborn Son of God with His many brothers as the many sons of God in the main line of God's economy (Rom. 6:14).