The Glagolitic alphabet is an invention rather than an adaptation. Its letters are not Greek letters with additions but an original set, and its inventory was built to the sounds of ninth-century Slavic instead of inherited from a language that lacked several of them. That is rare. Most scripts are borrowed and then patched. This one was designed, and designed for a particular job, which was to carry the Gospels and the liturgy.
The translation work began before the brothers left for Moravia and continued there. What came out of it is the language now called Old Church Slavonic, the first literary Slavic language, and it became the liturgical language of a large part of the Orthodox world.
The opposition came from the Frankish clergy already working in the region, and it was doctrinal as well as territorial. The argument, which later writers call the trilingual heresy, held that scripture and worship belonged properly to Hebrew, Greek and Latin, the three languages of the inscription on the cross, and to no other. Constantine argued against it at Venice and then at Rome, where Pope Hadrian II approved the Slavonic books and had them laid on an altar in the city. He did not leave Rome again. He took monastic vows and the name Cyril shortly before his death there on 14 February 869, and he was buried in the basilica of San Clemente.