Christian Unity as Addressed at Vatican II

From Lumen Gentium, Dogmatic Constitution on the Church of the Second Vatican Council:

15. The Church recognizes that in many ways she is linked with those who, being baptized, are honored with the name of Christian, though they do not profess the faith in its entirety or do not preserve unity of communion with the successor of Peter. (14*) For there are many who honor Sacred Scripture, taking it as a norm of belief and a pattern of life, and who show a sincere zeal. They lovingly believe in God the Father Almighty and in Christ, the Son of God and Saviour. (15*) They are consecrated by baptism, in which they are united with Christ. They also recognize and accept other sacraments within their own Churches [Orthodox Churches who adhere to the sacramental episcopacy] or ecclesiastical communities [the lay communities of Protestantism]. Many of them rejoice in the episcopate, celebrate the Holy Eucharist and cultivate devotion toward the Virgin Mother of God.(16*) They also share with us in prayer and other spiritual benefits. Likewise we can say that in some real way they are joined with us in the Holy Spirit, for to them too He gives His gifts and graces whereby He is operative among them with His sanctifying power. Some indeed He has strengthened to the extent of the shedding of their blood. In all of Christ's disciples the Spirit arouses the desire to be peacefully united, in the manner determined by Christ, as one flock under one shepherd, and He prompts them to pursue this end. (17*) Mother Church never ceases to pray, hope and work that this may come about. She exhorts her children to purification and renewal so that the sign of Christ may shine more brightly over the face of the earth.

A snippet from #3 of Unitatis Redintegratio, Decree on Ecumenism: 

Moreover, some and even very many of the significant elements and endowments which together go to build up and give life to the Church itself, can exist outside the visible boundaries of the Catholic Church: the written word of God; the life of grace; faith, hope and charity, with the other interior gifts of the Holy Spirit, and visible elements too. All of these, which come from Christ and lead back to Christ, belong by right to the one Church of Christ.

The brethren divided from us also use many liturgical actions of the Christian religion. These most certainly can truly engender a life of grace in ways that vary according to the condition of each Church or Community. These liturgical actions must be regarded as capable of giving access to the community of salvation.

I have deliberately highlighted passages here that affirm the universal role of baptism as the foundation for unity among Christians. The Catholic Church recognizes baptism as an act of God that is not limited to Catholics but indeed that incorporates every Christian into the body of Christ. The Catholic Church also recognizes as sacramental public marriage between two Christians (but public for Catholics means that it must be within the authority of the bishop). Although Protestant communion services are not sacramental, they are prayers that seek a deeper union with Christ and with other Christians.