There is an opinion among, at least some, Orthodox scholars at present that the limits of the Church can be defined in terms of baptism. Within this limit it is proposed that some division can exist, such as between Roman Catholic and Orthodox, and that we can still speak of the divided parties as Church. Thus, because most Christian groups/denominations/churches perform baptism they are all in some manner in the Church as distinct from non-baptised who are outside the Church. In practice this is the reasoning supporting marriage between those who are baptised even if they are of different creed and one of the partners would nevertheless be excluded from the eucharist of the other’s church.
It is agreed that baptism is the means of entry into the Church and that the members of the Church are identified as those that have been baptised, which permits them to gather with the other members in the eucharist. However, is the act of wetting someone with water while saying a particular formula in itself what baptism is? Is the Church defined as those people who have had undergone such an act? If so then once one has undergone that act then can they cease to be members of the Church regardless of faith and morals and choice?
The Church is the body of Christ, with Christ as the head. It is not merely the collection of those having participated in a particular form of ritual or believing a particular doctrine, although these may be marks of those in the Church, that is the Church is not a collection of those having certain marks and in particular of having one mark isolated from the others, which is the implication of the opinion stated initially. Rather the Church is those who form the body of Christ, that is those who are united with Christ, who is a living person. Those united to Christ come into the life of the Trinity as sons of God having the Spirit rest upon them that is they become deified.
Baptism brings us to membership of the Church because of a number of features in baptism. Considering baptism in water. Firstly, the water must be sanctified, it is not merely any water but that upon which hovers the Holy Spirit. Secondly, the triple immersing and coming out of the water unites us with Christ and the Son of God through his death and resurrection and three day burial thus identifying us as sons of God. Thirdly, the triple immersing into the three distinct names of the Trinity each invoked with an immersion, Father, Son and Holy Spirit brings us into the life of the Trinity. This is baptism in water but this in itself is insufficient because we must be born of water and Spirit, so one also requires the anointing of Chrism blessed by a bishop to symbolise the baptism of the Spirit and thus that the Spirit rests upon the baptised and confirms that he is both a son of God and has come into the life of the Trinity with God as his Father. The correct expression of faith is also another requirement of baptism that the person believes into the name of Christ; that they freely and willingly accept Christ and participation in the life of the Trinity. And another requirement is that of the baptiser, a bishop or presbyter, because is God who gives new birth to the baptised not man thus the bishop/presbyter manifests the Father in terms of the baptism so that the baptised in generated as son by the Father and the procession of the Spirit who rests on the baptised as a son of God. The whole Trinity in various symbols is manifest in the baptism. The bishop/presbyter are also the focal points of union with Christ, that is the baptism is the means for a person to enter a relation with Christ who is present in the bishop/presbyter and also that it is Christ who sanctifies the waters for baptism. It is this relationship that identifies one as a member of the Church, and this relationship can only be established through baptism. Thus, when one speaks of membership in the Church via baptism it means not only the application of water with a formula but a relationship with a hierarchy that confers the baptism as a gift to bring the baptised into relation with the hierarchy, through whom Christ is present and so the baptised enters relationship with Christ and into the life of the Trinity. Also, if Chrism is not that from the bishop then there is no baptism in Spirit and so the person who has been immersed in water with the appropriate formula is not born again of water and Spirit and so not yet a member of the Church.
If Christ is not present in the baptiser then the baptism is not of God and it is not one that brings the baptised into a relationship with Christ nor into the life of the Trinity, so the baptism is of no effect and does not permit entry into the Church. Baptismal membership is dependant on who baptises and not only the other symbols of baptism.
So, baptism as a means of showing the limits of the Church cannot be separated from showing the limits of the Church in terms of the hierarchy, which also defines legitimate eucharistic gatherings. This later limit is based on a mutual sharing of one faith/tradition, tracing ordination to the Apostles, and mutual recognition. It is quite wrong to speak of some form of Church membership by baptism that includes those of separated hierarchies, particularly where this is due to difference in faith/tradition. One separating from the hierarchy appointed by Christ and the Apostles is no longer united with Christ and so no longer with the Church, even if they were properly baptised. That is why marriage is only to be between people of the same faith, and whether one has a form of baptism within a group of differing faith is irrelevant to this issue, the canonical test is orthodoxy not baptism. The form of baptism conducted outside the union of faith only has relevance to the economy of receiving converts.