Why was everything fulfilled through Israel instead of going directly to the Gentiles?
Everything was fulfilled through Israel because God's plan centered on the Messiah first redeeming Israel. After Israel received her Redeemer, she was to pass that salvation out to the surrounding Gentile nations and serve as priests, rather than God going directly to the Gentiles.
Les explains that at Pentecost Peter addressed "Ye men of Israel," with nothing concerning the Gentiles. Jesus came to Israel performing miracles, wonders, and signs to prove He was the promised Messiah, the King ready to give them the kingdom, and their Redeemer. David's prophecies spoke of Christ's coming, death, and resurrection, but not yet Gentile salvation. The purpose was that everything could be fulfilled so He could be Israel's Redeemer, and Israel could pass it out to the nations around them and be priests.
For David speaketh concerning him, I foresaw the Lord always before my face, for he is on my right hand, that I should not be moved:
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