In view of the context Jesus" reference to fire must be as a symbol of judgment primarily rather than purification, its other common signification in Scripture. He had just spoken of judging unfaithful disciples ( Luke 12:45-48). Now He explained that one of the purposes of His incarnation was to bring judgment to the earth (cf. Luke 3:16). Perhaps Jesus wished this aspect of His ministry was taking place already because it would result in the purification of His people and would usher in the kingdom. However before Jesus" judging ministry could begin, Jesus Himself would have to undergo judgment, which He pictured as baptism. It would overwhelm Him, but only temporarily. He would rise from it as a person experiencing water baptism rises out of the water. The prospect of His baptism (i.e, the Cross) distressed Him because it involved bearing God"s wrath for the sins of humankind. [source][source][source]
John wrote that God did not send the Son into the world to judge the world ( John 3:17). He meant at His first coming. When Jesus returns at His second coming He will exercise judgment. [source][source][source]