Can an evil spirit or demon ever acknowledge that Jesus Christ has come in the flesh, or confess that Jesus Christ is come in the flesh, can they ever say that?
1 John 4:1-3 "1 Dear friends, do not believe every spirit, but test the spirits to see whether they are from God, because many false prophets have gone out into the world. 2 This is how you can recognize the Spirit of God: Every spirit that acknowledges that Jesus Christ has come in the flesh is from God, 3 but every spirit that does not acknowledge Jesus is not from God."
In fact, demons have recognized that the Son of God has come in the flesh. See the text below.
Matthew 8:28-34
Easy-to-Read Version
Jesus Sends Demons Out of Two Men
28 Jesus arrived at the other side of the lake in the country of the Gadarene[a] people. There, two men who had demons inside them came to him. They lived in the burial caves and were so dangerous that no one could use the road by those caves. 29 They came to Jesus and shouted, “What do you want with us, Son of God? Did you come here to punish us before the right time?”
30 Near that place there was a large herd of pigs feeding. 31 The demons begged Jesus, “If you make us leave these men, please send us into that herd of pigs.”
32 Jesus said to them, “Go!” So the demons left the men and went into the pigs. Then the whole herd of pigs ran down the hill into the lake, and all were drowned. 33 The men who had the work of caring for the pigs ran away. They went into town and told the people everything that happened, especially about the men who had the demons. 34 Then the whole town went out to see Jesus. When the people saw him, they begged him to leave their area.
John was writing to deal with gnosticism. Gnostics both today and back them deny that the flesh exists. That matter is all an illusion.
1 John 2:22
Easy-to-Read Version
22 So who is the liar? It is the one who says Jesus is not the Messiah. Whoever says that is the enemy of Christ—the one who does not believe in the Father or in his Son.
Gnostics also teach that Jesus is only a man and Christ was just an idea. Today they would say Christ Consciousness. The cult known as Christian Science and the mind sciences. Thus they deny that Jesus is the Messiah.
Gnostics both then and now, deny that sin exists.
1 John 1:8-10
Easy-to-Read Version
8 If we say that we have no sin, we are fooling ourselves, and the truth is not in us. 9 But if we confess our sins, God will forgive us. We can trust God to do this. He always does what is right. He will make us clean from all the wrong things we have done. 10 If we say that we have not sinned, we are saying that God is a liar and that we don’t accept his true teaching.