To be more precise, Glagolitic was the originally created script, with wholly original letter shapes. Cyrillic was a later iteration by the Slavs themselves -

To be more precise, Glagolitic was the originally created script, with wholly original letter shapes. Cyrillic was a later iteration by the Slavs themselves - Bulgarians, to be precise - where those new shapes were mostly replaced with Greek letters (except where there was no direct equivalent), presumably because those guys were translating a lot of Greek books, and having similar alphabets for both Greek and Church Slavonic made things easier.