It depends on whether one means by catholic, a Roman Catholic or a member of the then universally acknowledged Catholic church. If the former, then the answer is no. He was a member of the British Celtic church which acknowledged all other national churches as sisters, and this included the Church of Rome. Pelagiius was a Briton and a monk. He must have been ordained, as he was the twentieth abbot of Bangor Iscoed whose founder was always regarded as St Paul who, during the six years between his two imprisonments at Rome, visited and evangelised in Britain. Bangor's discipline and doctrine were certainly known as "the Rule of Paul". Its abbots regarded themselves as his successors; they were always men from the highest grade in society, and generally of royal blood. Bede and other authors state the number of monks in it at 2,100. Its scholars numbered in the thousands. St Hilary and St Benedict termed Bangor "the mother of all monasteries."
Pelagius may have been reacting to the strange doctrines beginning to arise within the Roman church and those churchs that were being influenced by them.
Sources: Vita Pelagii, p.3; Ancient British Triads; Triads of St Paul; RW Morgan, St Paul in Britain, p.118-20, ISBN 0 - 934666 - 12 -1.
CATHOLIC ANSWER
There are a few Pelagius': (1) Popes Pelagius I and II = both Catholics; (2) St. Pelageus 912-925 AD)-- a 13 year old Catholic boy, martyred in Cordoba Spain by the Muslims for refusing to renounce Christianity. He died after 6 hours of agony. There are other Pelageus' as well. The Pelageus the questioner is most likely referring to is #3.
(3) Pelagius (354 AD-420 AD; ascetic monk from Briton-likely Scotland or Ireland) & the 5th Century Pelagian heresy. Palagianism derives its name from this 5th Century monk, Pelagius.
In one of his Anti-Pelagian works, St. Augustine gives a brief overview of Pelagianism, stating that:
"…after the older heresies, there has been just now introduced, not by bishops or presbyters or any rank of the clergy, but by certain would-be monks, a heresy which disputes, under colour of defending free will, against the grace of God which we have through our Lord Jesus Christ; and endeavours to overthrow the foundation of the Christian faith of which it is written, "By one man, death, and by one man the resurrection of the dead; for as in Adam all die, even so in Christ shall all be made alive;" 1 Corinthians 15:21-22 and denies God's help in our actions, by affirming that, "in order to avoid sin and to fulfil righteousness, human nature can be sufficient, seeing that it has been created with free will; and that God's grace lies in the fact that we have been so created as to be able to do this by the will, and in the further fact that God has given to us the assistance of His law and commandments, and also in that He forgives their past sins when men turn to Him;" that "in these things alone is God's grace to be regarded as consisting, not in the help He gives to us for each of our actions,"- "seeing that a man can be without sin, and keep God's commandments easily if he wishes." (On the Proceedings of Pelagius, 61)
Sources:
Pope Pelagius II was the predecessor of Pope Gregory I, the Great.
Alvarus Pelagius was born in 1280.
Alvarus Pelagius died in 1352.
Pelagius of Cordova was born in 912.
Pelagius of Cordova died in 926.
Pelagius of Oviedo died in 1153.
Pelagius of Asturias died in 737.
Pope Pelagius II died in 590.
Pope Pelagius II was born in 520.
Pope Pelagius I died on 561-03-04.
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Original Sin