In common parlance, the words church and parish have come to be used almost completely interchangably. This imprecise habit becomes very problematic, however, when the emotionally-charged issue of altering the configuration of parishes arises in the life of a diocese. In as few words as possible: a church is a building set aside for sacred worship; a parish is a jurisdictional division of a diocese. And each have their proper names.
According to c. 1218, “Each church is to have its own title which cannot be changed after the church has been dedicated.” This can only be taken to mean the church building: the canon is placed squarely in the midst of the title of the Code dealing with sacred places, and so has nothing to do with parishes qua parishes. However, since clarity concerning this topic has come to be of increased importance in recent years, the Congregation for Divine Worship and the Discipline of the Sacraments issued norms on church titles in 1999.
The options for church titles are clearly laid out in the Rite of Dedication of a Church, in a list that is undoubtedly intended to be exhaustive:
- the Blessed Trinity;
- our Lord Jesus Christ invoked according to a mystery of his life or a title already accepted in liturgy;
- the Holy Spirit;
- the Blessed Virgin Mary, likewise invoked according to some appellation already accepted in the liturgy;
- one of the angels [or, it would seem safe to presume, all of them]; or, finally
- a saint inscribed in the Roman Martyrology or in a duly approved appendix.
Once the church has been solemnly dedicated under the chosen title, this title cannot be changed: c. 1218 is completely unambiguous about that. These norms offer the possibility of changing the title of a dedicated church, but only for grave reasons and with a specific indult from the Apostolic See. It is not, however, expressly stated any example of what might be a sufficiently grave reason, nor how disposed the Congregation would be to grant such an indult.
Church Titles, Parish Titles
Do parishes have titles, too? As juridic persons, they certainly can, although in most cases such a parish title would be identical to that of the parish church; and as civil corporations they certainly need to be called something in the eyes of the civil government. There is, however, no specific provision in canon law which either requires or regulates a parish name. So if the new parish church is dedicated to Saint Peter, the parish will most naturally be called the Parish of Saint Peter. While nothing in the law seems to prevent the juridic person of the parish from having a name that does not make reference to the title of the parish church, it is certainly traditional to do so.
And what is to be done when existing parishes are merged? According to the norms from CDWDS:
When a new parish has been erected in the place of several suppressed parishes, the new parish may have its own church which, unless it is a new building, retains its proper title. Further, churches of suppressed parishes, whenever such parishes are considered “co-parishes,” retain their own proper titles.
From this it seems very clear that, under ordinary circumstances, the CDWDS expects that parish churches in merged parishes will continue to have the same title with which they were dedicated. Yes, there is provision, as already mentioned, for the diocesan bishop to request an indult to change the tile for grave reasons, but from the wording of the passage just cited, the Congregation does not view the merging of parishes, in and of itself, to constitute such a grave cause. Instead, a new name for the parish may be chosen and used, while continuing to use the proper titles of the separate parish church(es):
If several parishes are joined so that a new parish is established thereby, it is permitted, for pastoral reasons, to establish a new name [for the parish] differing from the title of the parish church.
So, if there are multiple churches in the new parish, they could be referred to by their own proper titles along with the parish name, e.g. Our Lady of ____ Parish at St. _____ Church. While this will appear wordy, formal, and is potentially ungainly, precision in naming is almost always important — and helpful — to maintain.
 Congregation for Divine Worship and the Discipline of the Sacraments, Notification, Omnis ecclesia titulum, 10 February 1999, in Notitiae, 35 (1999), pp. 158-159; English translation in Roman Replies and CLSA Advisory Opinions 1999, Washington DC, Canon Law Society of America, 1999, pp. 17-21.
 See Roman Pontifical, Rite of Dedication of a Church and an Altar, II, 4.
 If a church has been only blessed, then the diocesan bishop can change the title, for reasons which are sufficiently grave and after having considered carefully all the circumstances (Omnis ecclesia titulum, no. 6; cf. CLSAComm2, p. 1431). Bear in mind, though, that c. 1217, §2 explicitly prescribes that churches — and “especially” parish churches — “are to be dedicated by the solemn rite.”
 Omnis ecclesia titulum, no. 5.
 Ibid., no. 11 (emphasis added).
 Ibid., no. 12.
 See John Huels, “Change of Title of Church,” in CLSA Advisory Opinions 1994-2000, Washington DC, Canon Law Society of America, 2002, pp. 392-393.