John (died 1147) was an early 12th century tironensian cleric. He was the chaplain and close confident of King David I of Scotland, before becoming bishop of Glasgow and founder of Glasgow Cathedral. He was one of the most significant religious reformers in the history of Scotland. His later nickname Achaius, a latinization of Eochaid would indicate that he was a native Scot, but the name is probably not authentic. He was in fact a Tironensian monk, of probable French origin, probably being taken to Scotland from David's lands in the Cotentin Peninsula.
While David was in the custody of King Henry I of England, he spent some time in northern France. David came to cultivate strong relations with the new Tironensian monastic order, and in 1113 established a Tironensian monastery at Selkirk Abbey. John may have either been the cause of this relationship, or perhaps its product. John was serving as David's chaplain until about 1116, and was appointed bishop of Glasgow sometime thereafter. John was involved in a dispute with the archbishop of York, a dipsute general to the David's kingdom. After the accession of Thurstan to the archbishopric of York, John received several letters from Pope Callixtus II ordering him to render homage to this archbishop as his metropolitan. In 1122 Thurstan suspended John, an action which was obviously serious enough for John to travel to Rome to appeal. Afterwards, John travelled on pilgrimage to Jerusalem, but in 1123 was ordered by the pope to return to his diocese. John travelled to Rome again in 1125 in order to secure a pallium, which would have elevated St Andrews to an archbishopric. Thurstan soon arrived in Rome himself. This was probably enough to prevent Pope Honorius II granting the pallium, and the year 1127 was set to continue discussion about the archbishop's rights, effectively stalling Thurstan's claims.
Nevertheless, York's claims continued to be pressed. In 1134, there was renewed papal pressure from Pope Innocent II to make submission. Perhaps it was for this reason that John gave his allegiance to the Antipope Anacletus II. The political situation had changed by 1135, and John's move had put him out of favour. In either 1136 or 1137 John abandoned his see to become a monk at Tiron. However, in 1138, the papal legate Alberic, bishop of Ostia recalled him to his see. John died in 1147, and was buried in Jedburgh Abbey. He was succeeded by another Tironensian, Herbert, Abbot of Selkirk/Kelso.
John's legacy was vast. His impact as a confident of David was crucial to the growth of reformed monastic orders in the kingdom of Scotland. Moreover, John himself presided over the monastic foundations of Selkirk (later Kelso Abbey, Kelso), Jedburgh and Lesmahagow. John's episcopate saw the beginnings of Glasgow cathedral.