To God, everything is possible. But let us explore this further.
What happens to us when we die?
Two schools of thoughts. Some believe we come face-to-face with God and we are judged and we go to either Heaven or hell. The Catholics also believe we go to Purgatory; but let us not confuse matters even more for now.
The options appear to be either Heaven or hell.
When Christ died on the Cross He said to the thief
by His side, "Today you will be in Paradise with
me". This does imply instant judgement at the time of
When our loved ones die, often at the funeral we
are told in sermons that they are in Heaven,
looking down on us.
When Christ went up the mountain with Peter, James and John, (The Transfiguration - Matthew 17:1–8, Mark 9:2–8, Luke 9:28–36), the disciples saw Moses and Elijah talking to Jesus. Now we assume that they saw them as bodies not just spirits. This means that God is able to let dead people appear again on earth as human bodies to suit His purposes.
So the question is: If God wanted to, can He allow a certain dead person to go back to earth to serve God's purpose? The answer is YES - God can do anything.
The next question then is: Does
God do that? Does He send people (souls) back to
earth to serve His purpose? We don't know the
answer, but I see no reason why He does not do
God sent Angels many times on earth to announce to Mary about Christ's birth, to announce to the shepherds the same thing, and to warn Joseph to go to Egypt. So why can He not send souls from Heaven to visit earth to serve His purpose - perhaps to protect us?
This raises another
question: Are Angels souls of those who have
died and gone to Heaven? Do souls in Heaven
somehow become Angels?
Whilst we have no answer to those questions, and they are not raised in the Bible, apart from the fact that Angels exist, we can assume that if God wanted to He could make souls in Heaven Angels, and He could send them to various missions on earth.
And another question. Are ghosts/spirits here on earth with God's express permission, to accomplish a mission perhaps?
When Christ walked on water, His disciples on
the boat thought they had seen a ghost. Jesus
did not say, "Hold on chaps. There are no such
things as ghosts!"
When He appeared to the disciples after the Resurrection, He said, "Look, it is me. I am not a ghost. I have flesh and bones. Ghosts do not have flesh and bones". (Luke 24:36 onwards - and other Gospels).
So this implies that ghosts do exist and they are spirits - not humans with flesh and bones.
In my view, and I'd appreciate yours: To God all is possible.
He could, if He wishes, send back the souls of dead people back to earth in the form of spirits or angels or ghosts. They would certainly not be able to do so without His express permission.
We will not know the answers to all these questions ... not now at least.