Where does the belief that humans become angels after death come from?
Who (which groups) have this belief? Is it an idea from other religions that has been incorporated into Christian beliefs? Has it happened to other religions as well?
As the above says, the Bible does not equate humans as angels in either the current or afterlife. In fact it says this of angels: “Therefore, angels are only servants--spirits sent to care for people who will inherit salvation” (Hebrews 1:14 - NLT).
It is a common claim of spiritualists though, who say that the spirits they come in contact with are former relatives who have passed away. It’s helpful to their cause and makes their practice look acceptable. But many others believe that these spirits may be demons who are trying to deceive the people into having a false idea of the afterlife.
The Bible though promises something different for humans who die. Jesus said, “My sheep hear my voice, and I know them, and they follow me: And I give unto them eternal life; and they shall never perish, neither shall any [man] pluck them out of my hand” (John 10:27,28 - NKJV). The only requirement he mentions is just to hear him and follow him, and anyone can do that.
There are conventional and canonical differences of notion in relation to this question. Not only does folk culture support the idea in some contexts that all sincere adherents becomes saints upon death, some also affirm that the worthy dead become angels. This distinction stratifies into "God's angels" or "archangels" with greater authority and appeal and suborders or subordinates of the angel caste.
Even within the subordinates themselves they may be identified as personal (or "guardian") angels for individuals, and these are stratified by some in the sense that a portion of these are deemed "without their wings" which are earned, like badges or power-ups. This can be seen in popular media where it is promoted that "Every time a bell rings an angel gets its wings."
Usually the angel caste is reserved for those who earn the status, and this is often restricted or designated for those who were part of the known religious body of the speaker or individual affirming the ideology (i.e. it is insular or chosen of those of one's own body of believers).