Their angels always see the Father's face

10 See that you do not despise one of these little ones. For I tell you that in heaven their angels always see the face of my Father who is in heaven (Mt 18:10). 

15 They said to her, “You are out of your mind.” But she kept insisting that it was so, and they kept saying, “It is his angel!” (Acts 12:15).

Commentators find the reference to "angels" in these passages somewhat puzzling, but they usually settle on guardian angels as the best identification (Carson is a notable exception.) However, the comparative literature they adduce for that identification is scant and tenuous. The closest analogy is the role of the Archangel Michael. However, he's the guardian angel of Israel. One can't extrapolate from that to a doctrine of guardian angels for every individual.

One problem is that Bible scholars, like other scholars, suffer from tunnel vision. They may know a lot about their area of specialization, but not much outside their specialty.

Given widely reported apparitions of the dead generally, as well as crisis apparitions in particular, I think it's more likely that Acts 12:15 reflects popular belief in apparitions of the dead. Belief that, at the moment of death, or shortly thereafter, the decedent may appear to friends and relatives. Or may appear to friends and relatives when they are undergoing a crisis. That fits the context of Acts 12 like a glove. On that view, Rhoda figured that Peter had died in custody, and this was his way of saying good-bye before he went to heaven.  

I'm not using this as a prooftext for apparitions of the dead. It reflects the viewpoint of a figure in the narrative (Rhoda), and not necessarily the viewpoint of the narrator (Luke). Moreover, Rhoda is not an inspired speaker or normative character.

Nevertheless, if this identification is correct, it presumably reflects popular folklore about ghosts and apparitions of the dead. And what would give rise to that belief? Well, maybe real encounters of that kind.

Assuming this interpretation is correct, it sheds light on Mt 18:10. If "angel" is sometimes a synonym for a ghost or apparition of the dead, then that refers to the souls of the departed. And if that's the correct identification, then this may be the most promising prooftext for universal infant salvation. 

An objection to that inference is that Jesus is using "little ones" as a metaphor for Christians. Since, however, he introduces his comparison by using a child as an object lesson, it would seem rather incongruous to exclude literal children from the tally when he presents them as the exemplary standard of comparison which Christians are required to emulate in that regard. But I admit this isn't a knock-down argument.