These early Christian 'Apostles of the Slavs' were brothers from Thessaloniki. It seems that although their father Leo was a higher ranking Byzantine soldier, their mother Maria was a Slav and hence all their seven children spoke Greek as well as some Slavic dialect.
Constantine, called The Philosopher, was a brilliant student at Constantinople and became a priest and a professor at the university there as well as a librarian at the basilica. He was very fond of St Gregory of Nazianzus, The Theologian, and he also spoke Arabic and Hebrew. He discovered the relics of Pope St Clement on his mission to the Khazars.
Methodius, who was born Michael, was first a provincial governor before joining a monastery on the Bithynian Olympus (north-west Turkey). He may have been an abbot there too.
In 863, upon an invitation by prince Rastislav, both brothers arrived to Nitra (in current Slovakia) in order to evangelise the Great Moravian Empire (which comprised at certain times of parts of the current Czech Republic, Slovakia, Slovenia, Hungary, Poland and Ukraine). In order to do this, they created a unique script called Glagolitic (a pre-cursor to the Cyrillic) which had the capacity to capture the unusual Slavic sounds. Using this script, they translated the Scriptures, some liturgical books and legal documents. However, the Latin clergy in the Frank (German) Empire were so opposed to this, that the brothers had to travel to Rome a few times to get a renewed papal confirmation for their work in the vernacular.
Eventually, Constantine became a monk there, taking the name Cyril. He died in Rome in 869 and was buried in the Basilica of St Clement.
Methodius, consecrated a bishop by Pope Adrian II, received permission to use the Slavic language for liturgical and canonical purposes in the new Slavic dioceses of Moravia and Pannonia. Thus, in 9th century, the (old) Slavonic was the only other liturgical language permitted in the Catholic Church - after Latin, Greek, and Hebrew. Archbishop Methodius then returned to the Great Moravia to continue his work but he was imprisoned and tortured for three years by the Frankish clergy who disliked the creation of the independent Slavic territories.
Methodius was liberated by the newly elected pope John VIII, but he died ill and exhausted in 885 after another visit to Rome. He named one of the Slavic clergy (Gorazd) as his successor, but all his disciples were heavily persecuted upon Methodius’ death by Wiching, the Frankish bishop, and ultimately exiled. Latin language and script were swiftly reestablished in the Great Moravian territories, while Cyrillic (in honour of St Cyril) was developed from Glagolitic and Greek in Bulgaria by the run away disciples.
The brothers were declared co-patrons of Europe with St Benedict of Nursia in 1980. The Macedonian brothers are also patrons of unity and ecumenism.
(Many thanks to Monika Chmelova for providing this text)