Remember Christ and where He came from. Paul called on the Gentiles to remember their past prior to, or without Christ.
1) “Alienated from the commonwealth of Israel” In the O.T. the people of God were the nation of Israel.He had made a covenant with them - to be their God. In the N.T. the people of God are the church - the bride of Christ. With whom God has also made a covenant with - that nothing will be able to separate us from his love.
2) “Strangers to the covenants of promise.” When separated from Christ, interacting with the covenant promises of God would make them feel like they were missing out on something.