Many people believe they have a guardian angel but reject Christ.
They say they have a guardian angel and truly believes in it, but they don't realize it is an evil spirit, for only Christians have guardian angels. Paul said that Satan appears as "an angel of light" (2 Cor. 11.14).
We can apply these verses to show it.
"You're out of your mind," they said. When she insisted, they decided, "It must be his angel." (Acts 12.15)
Ask the person if the angel they speak of is a guardian angel and is of God, why does it deny the Christ? It therefore, must be a fallen angel or a demon in possession. There is no other option.
"Now the Spirit expressly says that in latter times some will depart from the faith, giving heed to deceiving spirits and doctrines of demons" (1 Tim. 4.1).
"But if anyone denies me here on earth, I will deny that person before God's angels" (Luke 12.9).