What is the baptism of fire?

The burnt offering, as a shadow of Christ's total devotion even unto death, makes the fire the essence of his tribulation. Christ's tribulation was not heaped upon him by mankind, but by God himself. The Holy Spirit, is Holy. When Christ bore the sins of mankind, even being made to be sin for us, how else should the Holy Spirit respond? Mankind nor the devil had any right nor ability to put him in tribulation. God himself was being satisfied for the sins of man.

So the fire represents the Holy Spirit, but it is not redundant. The Holy Spirit in heaven gives life, and the Holy Spirit on earth (fire) refines us. Jesus went through tribulation to learn obedience:

   Heb 5:8 Though he were a Son, yet learned he obedience by the things which he suffered;

Tribulation is the other half of baptism into life:

   Joh 16:33 These things I have spoken unto you, that in me ye might have peace. In the world ye shall have tribulation: but be of good cheer; I have overcome the world.

Jesus said that in the midst of the fire, he would be with us:

   Da 3:25 He answered and said, Lo, I see four men loose, walking in the midst of the fire, and they have no hurt; and the form of the fourth is like the Son of God.

Someone might object that Nebuchadnezzar caused the tribulation, but even Satan needed permission to torment Job.