God’s sovereign election has to be matched by human responsibility.
Paul told the Gentiles not to be proud or complacent that they have been chosen (God's sovereignty) over the Jews. It's because the latter rejected God. The Gentiles have to realise there is a condition to be met if they want to remain in God's favour (human responsibility). Otherwise, they too will be cut off (Romans 11: 17-22).
"Note then the kindness and the severity of God: severity toward those who have fallen, but God’s kindness to you, provided you continue in his kindness. Otherwise you too will be cut off" (Romans 11:22).
It is a doctrinal error to emphasise the love of God over the justice of God. Both are attributes of God of equal standing. Furthermore, God is impartial in His dealings with people, whatever their ethnic group.
God in His sovereignty chose Paul, a great persecutor of the church, to be His instrument to bring the gospel to the Gentiles. Paul cannot claim any merit for being God’s chosen one. But he certainly lived up to his heavenly calling after the life-changing power encounter with God (Acts 26:19).