Vincent’s Word Studies has this on the “one baptism” of Ephesians 4.5, and why baptism but not the Lord’s supper is mentioned:

The external sign of faith, but of no significance without the Lord and the faith. Baptism is emphasized instead of the Eucharist, because the latter assumes and recognizes unity as an established fact; while faith and baptism precede that fact, and are essential to it. Baptism, moreover, is not administered to the Church as a body, but to individuals, and therefore emphasizes the exhortation to each member to be in vital union with the whole body.

Six of the nine times the word church appears in Ephesians—the theme of the letter—they occur in the passage discussing the marriage relationship, chapter 5. So what does this mean?