Is the devil God's eldest son?

No! God exists as: Father, Son and Holy Spirit. The Son came to this world as a man, so that He could be able to die, as a man, for our sins. His name on earth was Jesus.

 The devil was a beautiful angel in heaven who, filled with pride, opposed God. He became the evil one, the devil. He is not God's eldest son. We must be careful not to be misled by this notion.

 The devil was originally created as a good angel, to serve God. But after that he became proud, and rebelled against God's authority (1 Timothy 3:6-7). Then he was thrown out of heaven with the angels which followed him. The devil is evil, and in God there is no evil whatsoever.

 Eventually the devil will be punished in a lake of fire for all eternity (Revelation 20:10).

 Unfortunately it will not only he who will suffer. The Bible continues, "But the cowardly, the unbelieving, the vile, the murderers, the sexually immoral, those who practice magic arts, the idolaters and all liars -- their place will be in the fiery lake of burning sulfur. This is the second death." We pray that you will not be one these unfortunate souls, because you accepted Christ as your Saviour while you still had the time!