Of course that the Slavs knew literacy before Cyril and Methodius. The two of them were actually invented the Glagolitic alphabet. Glagolitic alphabet contains some characters from letters of northern Africa, which are known to Cyril and Methodius, but not the Balkan Slavs. This indicates that the letter they are made. Sami Slocveni used their old letter alphabet. Church authorities wanted at all costs to suppress the alphabet and replace the Glagolitic alphabet, to the Slavs clipped roots. Of course alphabet is the oldest letter because they are perfect and because it is universal. From it are later developed phonetic letters of the old world.