"When [Peter] realised this, he went to the house of Mary, the mother of John whose other name was Mark, where many were gathered together and were praying. And when he knocked at the door of the gateway, a servant girl named Rhoda came to answer. Recognizing Peter’s voice, in her joy she did not open the gate but ran in and reported that Peter was standing at the gate. 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!' But Peter continued knocking, and when they opened, they saw him and were amazed." - Acts 12:12-16
It is not wise to build one's theology upon a single instance in the book of Acts. Acts is descriptive, not proscriptive, narrative, not normative. So we must be careful here.
However, here the believers gathered at the house of Mary seem to believe Rhoda has seen Peter's "angel." It is unclear whether they mean an angel assigned to guard Peter or that Peter has died and himself become an angel. The latter option seems to be very bad theology. There is no indication elsewhere in Scripture that believers become angels when they die. Rather, the soul and spirit of a believer separates from his or her physical body and goes to be with Jesus, waiting for his or her resurrection body. Our loved ones are not angels watching over us. It's a nice thought...just not true.
The other option is that the believers gathered at Mary's house are talking about Peter's guardian angel, who may in some way look or sound like Peter. There are two cross references that seem to support the idea every believer is assigned a guardian angel.
"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." - Matthew 18:10
"Are [angels] not all ministering spirits sent out to serve for the sake of those who are to inherit salvation?" - Hebrews 1:14
These three passages, taken together, suggest every believer has a guardian angel watching over him or her. These guardian angels see the face of God and may be sent by him to intervene in our lives and to minister to us in various ways, usually unknown to us.
Father, this world you have made is incredible, and there are aspects of it that are hidden from our eyes. We do thank you for Scripture, which gives us everything we currently need to know about the unseen realm. Thank you you use angels to guard us and to minister to us. Be with your people today. In Jesus' name, amen.
- Jeff Coleman