How does the Church view the relationship of non-Catholic Christians with itself?

All men are called to be part of this Catholic unity of the people of God….And to this belongs or are in various ways related to it, the Catholic faithful, all who believe in Christ, and indeed the whole of mankind, for all men are called by the grace of God to salvation.“ (LG 13).

”They are fully incorporated in the society of the Church who, possessing the Spirit of Christ accept her entire system and all the means of salvation given to her, and are united with her as part of her visible bodily structure and through her with Christ, who rules her through the Supreme Pontiff and the bishops. The ties which bind men to the Church in a visible way are profession of faith, the sacraments, and ecclesiastical government and communion. He is not saved, however, who, though part of the body of the Church, does not persevere in charity. He remains indeed in the bosom of the Church, but, as it were, only in a "bodily" manner and not "in his heart." (LG 14).

The Church recognizes that in many ways she is linked with those who, being baptized, are honored with the appellation of Christian, though they do not profess the faith in its entirety or do not preserve unity of communion with the successor of Peter. (LG 15). "He who believes in Christ and has received Baptism in the correct manner, is in a certain, albeit incomplete communion with the Catholic Church“ (UR 3). Communion with the orthodox churches is so deep that only very little is missing to achieve the fullness required to permit the joint celebration of the Eucharist.“ (Pope Paul VI, Sermon of December 14th, 1975) (see also KK 836-838).