Where did the idea that people who die get turned into angels come from?

no clue about the after u die part - but when one becomes a believer he/she is called an Angel because the word Angel translates to "messenger" so who ever spreads the word of Christ is considered His Angel - and in Revelations the Pastor or Bishop of the churches that were named were called "The Angel of The Church .....that's the best way I can answer you - u don't turn into an angel in Heaven - Angels that are in Heaven were created by God as agents to fulfill his will without question and when an Angel disobeys it automatically becomes a Demon - we as humans were created differently: as a part of God - and as having freewill - I have no desire to be an Angel cuz being human is a much sweeter deal - in creation order Angels are higher - in authority humans are higher