When we think about world missions and reaching the unreached people with the gospel, when we think about being salt and light to our city, when we think about loving our neighbors in a way that benefits their souls, we should not underestimate the centrality of the Lord’s Supper.
Communion is a meal of victory and harmony. When we eat the bread and drink the wine by faith, we remember the Lord’s death, and victory over sin, until He returns, and His final victory over ever enemy. Also, when we eat and drink together, we share as one body of Christ and in one cup of blessing.
Paul quoted Psalm 18:49 in Romans 15:9. It is the first of four quotations in a row (two Psalms, one Deuteronomy, one Isaiah), all making the point that the hope of the Gentiles is in Christ.
But before and after those verses, Paul refers to our receiving one another.
May the God of endurance and encouragement grant you to live in such harmony with one another, in accord with Christ Jesus, that together you may with one voice glorify the God and Father of our Lord Jesus Christ. (verses 5-6)
May the God of hope fill you with all joy and peace in believing, so that by the power of the Holy Spirit you may abound in hope. (verse 13)
When we receive one another as Christ receives us (Romans 15:7), when we live in harmony with each other, and have hope that God Himself will make us one, we are giving reason for the Gentiles to glorify God for His mercy.