Belief in the Holy Trinity (the expression of God as incorporating three Persons in one God: Father, Son and Holy Spirit) is one of the defining elements of orthodox Christian belief, from early Christian times until the present; it is encoded in the Creeds which have been present in Christian churches since the 4th century Nicaean Creed. Despite this, it is only in the later middle ages, from perhaps 1100 on, that we begin to see the expansion of dedications of churches to the Trinity. The prompt for this expansion probably relates to the influence of Canterbury, with its twin dedication to Christ and the Trinity. Matthew Hammond has discussed the adoption of this dedication both by new foundations and older ones in twelfth- and thirteenth-century Scotland (Hammond 2010, 68-71).
Because of this, we should certainly see the presence of the Trinity in a dedication in North Uist at Teampull na Trionaid, as a later medieval development. As discussed in the entry on the site, the traditions of the church here in any case attribute its foundation to no earlier than the 12th century, and more frequently to the 14th century.
The distribution of commemoration of the Trinity in place-names underlines not only the lateness of this phenomenon, but the extent to which Teampull na Trionaid is isolated in a commemoration otherwise associated with the lowlands of Scotland. The explanation is probably to be found in the fact that, from the 14th century, this church, as capella Sancte Trinitatis, was given to Inchaffray Priory in Perthshire (Inchaffray Charters, 136-7). Although itself under the patronage of St John the Evangelist, Inchaffray also had a nearby church of the Trinity at Gask, also ecclesia Sancte Trinitatis, now Trinity Gask (DoSH), from at least the 13th century (Inchaffray Charters no. 45).
It is worth noting one small feature here. The parish of Trinity Gask is noted as Gascrist, that is *Gask Christ, in Gaelic, Gasg Chrìosd, in Bagimond’s Roll in 1275. This suggests that there was a dual dedication to Christ and the Trinity; or, rather, that the same dedication could be formulated in two different ways, as “the church of the Holy Trinity of Gask” or “the parish of Gask Christ”. This is not the only place this double-naming is found in Trinity dedications. Kirkdomine in Ayrshire is likewise capella Sancte Trinitatis de Kyldomine in 1391, and the Gaelic name seems to incorporate the Latin name of the Lord as Dominus (genitive Domini). In the Isle of Man, two churches which incorporate Christ in their Gaelic names, Kirk Christ Lezayre and Kirk Christ Rushen both seem to be dedicated to the Holy Trinity, with both having forms in sancte Trinitatis in the 16th century (Broderick 2006, 194). As noted above, Christchurch, Canterbury, which is perhaps the ultimate origin of this trend in dedications, is also dual dedicated to the Holy Trinity (see Hammond 2010, 68-71).
This is worth mentioning purely because of the proximity of Teampull Chrìosd and Teampull na Trionaid in North Uist, and because Teampull Chrìosd is also on lands belonging to Inchaffray Priory in the 14th century. There may thus be some other linkage between these two dedications.