St. Paul on Ecumenism:

While there is jealousy and rivalry among you, are you not of the flesh, and behaving in an ordinary human way?
Whenever someone says, "I belong to Paul," and another, "I belong to Apollos," are you not merely human?
What is Apollos, after all, and what is Paul? Ministers through whom you became believers, just as the Lord assigned each one.
I planted, Apollos watered, but God caused the growth.
Therefore, neither the one who plants nor the one who waters is anything, but only God, who causes the growth.
The one who plants and the one who waters are equal, and each will receive wages in proportion to his labor.
For we are God's co-workers; you are God's field, God's building.
(1 Cor 3:4-9)