1. Baptism is the bond of Christians, and thus the ground of catholicity (Eph. 4:4-6).
As a foundational post, let me quote that crazy ecumenist A.A. Hodge: “Above all the narrow, meagre patriotism on earth is the large, free, ecumenical patriotism of those who embrace in their love and fealty the whole body of the baptized. All who are baptized into the name of the Father, and of the Son, and of the Holy Ghost, recognizing the Trinity of Persons in the Godhead, the incarnation of the Son and his priestly sacrifice, whether they be Greeks, or Arminians, or Romanists, or Lutherans, or Calvinists, or the simple souls who do not know what to call themselves, are our brethren. Baptism is our common countersign. It is the common rallying standard at the head of our several columns. It is our common battle-flag, which we carry forward across the enemy’s line and nail aloft in the heights crowned with victory. We will be confined in our love and allegiance by no party lines. We follow and serve one common Lord. Hence there can be only ‘one Lord, one faith, one baptism,’ and hence only one indivisible, inalienable ‘sacramental host of God’s elect,'” (Evangelical Theology, 338).