Different ways of looking at the past - some like to reflect on the past while some 'bulldoze' the past.
Christianity is deeply connected to the past (eg repeating the creeds), and even connected to the Jewish nation.
1. Has God rejected Israel? Paul was a Jewish believer, and God told Elijah He had His people (remnant) (1 Kings 19). God saves the remnant by grace. Many do reject, but we must be like Paul in Acts 18:9-11. God knows His own. Paul didn't despair because God has a remnant.
2. God's methods. v7 Israel's rejection results so now sinners can have a perfect sacrifice (through Jesus), and their further rejection meant the Gospel spread to the Gentiles. This was always God's plan. And maybe this will wake up the Jews. God has always purposed one people - Jew and Gentile in Christ. V19-21 - Don't sneer at Jesus, as we too can lost. The Jews need also to be saved by faith in the Messiah, Jesus.
3. Implications: (a) Our Church is part of God's universal plan. (b) God knows who are His. (c) Don't write anyone off who is not a Christian. (d) Paul envisaged a Church that aroused envy in unbelievers. (eg, by forgiveness, saying we are sorry, welcoming, kindness etc) (e) Continue in grace (v22) (f) God does everything for His glory.
