1248 he
returned to Cologne with Albertus, and was appointed second lecturer and
_magister studentium_. This year may be taken as the beginning of his
literary activity and public life. Before he left Paris he had thrown
himself with ardour into the controversy raging between the university
and the Friar-Preachers respecting the liberty of teaching, resisting
both by speeches and pamphlets the authorities of the university; and
when the dispute was referred to the pope, the youthful Aquinas was
chosen to defend his order, which he did with such success as to
overcome the arguments of Guillaume de St Amour, the champion of the
university, and one of the most celebrated men of the day. In 1257,
along with his friend Bonaventura, he was created doctor of theology,
and began to give courses of lectures upon this subject in Paris, and
also in Rome and other towns in Italy. From this time onwards his life
was one of incessant toil; he was continually engaged in the active
service of his order, was frequently travelling upon long and tedious
journeys, and was constantly consulted on affairs of state by the
reigning pontiff.

In 1263 we find him at the chapter of the Dominican order held in
London. In 1268 he was lecturing now in Rome and now in Bologna, all the
while engaged in the public business of the church. In 1271 he was again
in Paris, lecturing to the students, managing the affairs of the church
and consulted by the king, Louis VIII., his kinsman, on affairs of
state. In 1272 the commands of the chief of his order and the request of
King Charles brought him back to the professor's chair at Naples. All
this time he was preaching every day, writing homilies, disputations,
lectures, and finding time to work hard at his great work the _Summa
Theologiae_. Such rewards as the church could bestow had been offered to
him. He refused the archbishopric of Naples and the abbacy of Monte
Cassino. In January 1274 he was summoned by Pope Gregory X. to attend
the council convened at Lyons, to investigate and if possible settle the
differences between the Greek and Latin churches. Though suffering from
illness, he at once set out on the journey; finding his strength failing
on the way, he was carried to the Cistercian monastery of Fossa Nuova,
in the diocese of Terracina, where, after a lingering illness of seven
weeks, he died on the 7th of March 1274, Dante (_Purg_. xx. 69) asserts
that he was poisoned by order of Charles of Anjou.