In 863, they began the task of translating the Bible into the language now known as Old Church Slavonic and travelled to Great Moravia to promote it. They enjoyed considerable success in this endeavour. However, they came into conflict with German ecclesiastics who opposed their efforts to create a specifically Slavic liturgy.
For the purpose of this mission, they devised the Glagolitic alphabet, the first alphabet to be used for Slavonic manuscripts. The Glagolitic alphabet was suited to match the specific features of the Slavic language and its descendant alphabet, the Cyrillic Alphabet, is still used by many languages today. [Wikipedia]