Where -- in the Bible does it warn us of Satan personating Christ?
Isaiah 14 shows it is Satan's aim.
14:13 For thou (Satan/Lucifer) hast said in thine heart, ... I will exalt my throne above the stars of God: I will sit also upon the mount of the congregation, in the sides of the north:

The sides of the north refers to Mount Zion, the place where the people came from all over Israel, to worship God in Jerusalem. Satan aims that he will also sit there.

Psalms 48:2 Beautiful for situation, the joy of the whole earth, is mount Zion, on the sides of the north, the city of the great King.

1 John 4:1 Beloved, believe not every spirit, but try the spirits whether they are of God: because many false prophets are gone out into the world.

Matt. 24:4-5 And Jesus answered and said unto them, Take heed that no man deceive you. For many shall come in my name, saying, I am Christ; and shall deceive many.

Matt. 24:23-24 Then if any man shall say unto you, Lo, here is Christ, or there; do not believe it. For there shall arise false Christs, and false prophets, and shall show great signs and wonders; insomuch that, if possible, they shall deceive the very elect.

In 1 Timothy 4:1 , the Apostle Paul warns, "Now the Holy Spirit clearly says that in later times some will abandon the faith and follow deceitful spirits and the teachings of demons."

2 Corinthians 11:13-15 , Paul warns of "false apostles, deceitful workers, masquerading as apostles of Christ. And no wonder, for Satan himself masquerades as an angel of light."

The Holy Spirit is essential in guiding believers into all truth and providing discernment and wisdom to recognize and resist deceitful spirits. In John 16:13 , Jesus promises, "But when He, the Spirit of truth, comes, He will guide you into all truth."

The Bible is essential in outlining truth and warning against deceptions.
2 Timothy 3:16-17 All scripture is given by inspiration of God, and is profitable for doctrine, for reproof, for correction, for instruction in righteousness: That the man of God may be perfect, thoroughly furnished unto all good works.

2 Peter 1:21 For the prophecy came not in old time by the will of man: but holy men of God spake as they were moved by the Holy Spirit.

Isaiah 8:20 To the law and to the testimony: if they speak not according to this word, it is because there is no light in them.

So how will God's people know those evil spirits masquerading as messengers of God, or even as Christ Himself, are NOT our Savior Jesus Christ, nor any heavenly messenger representing Christ, but are in reality deceiving fallen angels?

Isaiah 8:20 To the law and to the testimony: if they speak not according to this word, it is because there is no light in them.



It will be a powerful representation, how can people tell he's fake, what will be a distinguishing sign?



No matter how great the "wonders", even if appears like "fire coming down from heaven" Rev. 13:13. If he speaks not according to the law and to the testimony of God's Word, there is no light in him, only darkness.