It may be unexpected, and yet it is an obvious argument for justification by faith that God is one (Romans 3:30). Monotheism isn’t merely a guard against idolatry, it is also a guard against self-righteousness. Self-righteousness might be seen in man’s attempted defense before God, self-righteousness can be seen in man’s ethnic hatred and divisions. But God is one, His way of counting righteousness is one: by faith, and so no one can boast his way into separation from others.
Our communion as God’s people is monotheistic. That means that there is only one God, that we have peace with that God, and that we give our worship and thanks to no other.
Our communion as a church is also trinitarian. We worship one God in three Persons. We are baptized into the “name,” singular, “of the Father and of the Son and of the Holy Spirit” (Matthew 28:19). This fits with the reality that God is love (1 John 4:16), and that different persons can share equal status and not share every attribute equally. The Father never took on flesh, nor did the Spirit. The Son made propitiation, and it was His Father’s plan.
We share the same God. We share the same way of peace with God. We are not welcome at the Lord’s Table differently, due to our different works, because then we might boast. But we are also welcome at the Lord’s Table as different persons, in love, united as different members of one body (Romans 12:4-5).