The word profession has changed in its meaning. It originally meant, broadly, 'to admit knowledge of', or 'to announce loyalty to'. In English, the word was first used with a religious meaning. A Christian monk or nun would be a prfessed member of a particular order. Martyrs professed their faith in God by dying. This meaning was the only one, until the sixteenth century.
After the Reformation, the noun profession developed to mean one of the three learned professions (Theology, Law and Medicine). These required their members to have studied at university, hence the adjective learned. Nowadays, there are many more than three professions. The word can be used to mean any form of career, although it still carries connotations or requiring a higher degree of education than most forms of purely manual work.
There is a note on spelling at profession - professional - professionally - professor which you may want to see.