From Dom Guéranger's The Liturgical Year.
ST KENTIGERN, whose feast is kept to-day in several dioceses of the North of England and in Scotland, stands out as one of the zealous monks of the sixth century who laboured incessantly for the conversion of the inhabitants of these islands. He was brought up in the monastery at Culross from early childhood, and was thus trained from his youth in all the practices of monastic observance. However, when he reached man's estate, feeling called to a more rigorous manner of life, he left Culross and took up his abode in a solitude in the neighbourhood of Glasgow, where he led a most mortified life, eating but once in three or four days, spending the night in prayer, and wearing a garment of haircloth. This manner of life gave the greatest edification to all who came in contact with him, and his virtue was such that at the age of twenty-five he was elected to the bishopric of Glasgow. He had grave misgivings as to the validity of his ordination on account of his age, but was forced to bow before the importunities of those who had chosen him for their pastor. He ruled his vast diocese, which was bordered by the North Sea on the east and by the Atlantic Ocean on the west, with wisdom and prudence, though his manner of living remained unchanged, and, moreover, each Lent it was his custom to retire into solitude, giving himself entirely to prayer and living upon roots and herbs. He was assiduous in visiting his flock, travelling always on foot, and the force of his preaching together with the mildness and sweetness of his character and the asceticism of his life was the means whereby innumerable pagans were brought into the Church and many Pelagian heretics converted to the true Faith. This devoted prelate was the father of many monks, a great number of whom he sent to evangelize the north of Scotland, the Orkney Isles, and even Norway and Iceland.
The enemy of mankind, however, would not suffer so many souls to be snatched from his grasp without molestation, and he caused the royal family to raise such a bitter persecution against the saint that he was forced to leave the country. He took refuge in Wales, first at Caerleon, now Usk, where he built a church, then with St David, and finally he settled at the junction of the Rivers Elwy and Cluid, where he built a monastery, now known as St Asaph's. One of the princes of the country opposed this undertaking, whereupon he was struck with blindness, but was cured by the intercession of St Kentigern and thereafter became his great benefactor.
The great crime committed against St Kentigern in Scotland was not permitted to remain unpunished. All those who had persecuted the saint were visited by the just vengeance of Almighty God, and a prince, virtuous and loyal to the Church, ascended the throne, and his first act was to recall St Kentigern, who returned bringing with him some monks from St Asaph's. In 593 the Saint visited Pope St Gregory the Great and unburdened his soul of the doubts he had always held regarding his ordination. The supreme Pontiff set his mind at rest, and confirming him in office sent him back to his see, which he governed in peace for eight years more. On January 13, 601, he was called to his eternal reward at the advanced age of eighty-five, and was buried in his Cathedral Church at Glasgow.
The following is an abridgement of the account given of St Kentigern in the Breviary lessons for his feast.
Kentigernus, quem Scoti propter innocentiam morumque suavitatem, Munghum, id est, valde dilectum, appellarunt, ex regali Pictorum genere in Britannia Septentrionali ortum duxit. Adhuc puer monasterio Culross traditus, sub sancto Servano non minus litterarum quam rerum divinarum studiis mirabiliter profecit. Inde in solitudinem secessit apud Glascuam in Scotia, ubi vitam asperrimam in continua oratione rerumque cælestium meditatione traduxit. In episcopum delectus, et ad pastoralem dignitatem evectus, quasi lucerna supra candelabrum positus, apostolicis virtutibus statim inclaruit. Ejus prædicationem Deus multis magnisque miraculis roboravit, ita ut sanctus præsul, potens opere et sermone, a Pelagiana hæresi gregem suum servaret incolumem, innumeramque paganorum multitudinem Christi Ecclesiæ adjungeret. Ipse autem ab impio quodam tyranno exsulare in Walliam coactus, ibique, apud sanctum David episcopum aliquamdiu commoratus, mox ad fluenta Elwi et Cluidæ celebre fundavit monasterium, in quo sanctum Asaphum discipulum habuit. Tandem sæculo septimo, plenus dierum ad Deum migravit; cujus corpus, Glascuæ in ecclesia cathedrali conditum, ibi in maxima veneratione fuit, usque ad tempora, quibus sectæ Calvinianæ furor Catholicam fidem a Scotia pene exterminavit.
Kentigern, whom the Scots, on account of his innocence and sweetness of disposition, called Mungo, that is well-beloved, was of the royal family of the Picts of Northern Britain. While still a boy he was sent to the monastery of Culross, where, under St Servanus, he made great progress both in secular and in religious learning. Thence he withdrew into solitude, near Glasgow in Scotland, where he led an austere life of continual prayer and meditation upon heavenly things. He was elected bishop, and when he was thus raised to the pastoral charge his virtues shone forth as from a candle set upon a candlestick. God confirmed his preaching by many and great miracles, so much so that the holy bishop, mighty in word and deed, kept his flock safe from the Pelagian heresy and added a countless multitude of pagans to Christ's Church. A certain impious tyrant banished him to Wales, where, after having spent some time with the holy bishop St David, he founded a celebrated monastery at the junction of the rivers Elwy and Cluid, where he had as his disciple St Asaph. At length, in the seventh century, full of days, he slept in the Lord. His body was buried in the cathedral church of Glasgow, where it was held in great veneration until the time when the fury of the Calvinists almost extinguished the Catholic Faith in Scotland.