The Davidic Covenant refers to the promise of God to David, revealed by the prophet Nathan. It is an unconditional covenant, made between God and David, through which God promised David and Israel, that the Messiah (Jesus Christ) would come from the lineage of David and therefore, from the tribe of Judah, and His kingdom would endure forever (2 Sam. 7:10-13; 1 Chron. 17:11-14; 2 Chron. 6:16). The Davidic Covenant is unconditional because God does not place any conditions of obedience upon its fulfillment. The surety of the promises made rests solely on God’s faithfulness and does not depend at all on David or Israel’s obedience.
Because the covenant was with Israel, and not with the Church, and since the Church, as we know it, will no longer exist on earth during Israel's kingdom, I wondered how our offering would be used "to increase the kingdom." After all, the Church will be removed prior to the Tribulation, a period known as "the Time of Jacob's Trouble," that is, the final seven years of appointed unto Daniel's people, the Jews (1 Thes. 4:13-18; Jer. 30:7; Dan. 9:24-27).
I believe the answer lies in the mission of the Church here on earth, and its mission during the Millennial reign of Christ. Here, we are to take the Gospel message to the whole world; that includes to the Jews (Mt. 28:19-20). The majority of the Jews will reject our Lord, but we will have planted the seed, the Word of God (Mt. 13:19), and one day, right in the middle of the seven-year Tribulation (Dan. 9:27; Mt. 24:15; Rom. 11:25), that seed will burst forth in faith. The funds used to spread the Gospel will increase the number of Jews who accept Christ.
A second way the funds will increase Christ's kingdom, is as it wins Gentiles to Christ, those saved will rule and reign with Christ in their glorified bodies. They will be changed as they are being taken up in the Rapture (1 Cor. 15:50-54), and they will be ever present with the Lord (2 Cor. 5:8)! So, while the saved Jews will be members of God's kingdom, born again Christians will be administers in that same kingdom (Jn. 14:1-3; Rev. 5:10; 20:4).
Giving supports the spreading of God's Word, the power of God unto salvation!