Jesus' words here are quite strong. Let us elaborate upon them at this time. When our Lord declared that He came to "send fire on the earth" and wished that it was already kindled, it appears that "fire" is being compared to "division" and contrasted with "peace" (cf. 12:51). Thus, it is reasonable to affirm that the "fire" which Jesus speaks of here is the divisive (and purifying) effect that His teachings would have as His disciples preached the central message of the gospel in the future. In other words, when His disciples preached His death, burial, and resurrection (cf. I Cor. 15:1-4), the message would spread like a fire in the first century. Jesus came to Earth to save the lost, and the spread of the gospel message (i.e., the "fire") would be the means to accomplish it (cf. Luke 19:10; Mark 16:15,16).
However, before that would come to pass, Jesus would first have to suffer on the cross! His experience in being crucified can be properly understood as a "baptism" when one understands that to baptize a person or a thing simply means to immerse it, submerge it, or overwhelm it in some way. Jesus would be immersed into suffering at Calvary. He would be completely submerged into grief and anguish and be overwhelmed by it! Our Lord was not looking forward to the physical and spiritual pain He would endure, and that is why He described Himself as being "distressed...till it is accomplished". Certainly if it were possible to get the "fire" spreading without having to be immersed into agony on the cross, Jesus would have taken the alternative course of action (cf. Matt. 26:39). Jesus' wish at that moment was that Calvary was already behind Him!
Jesus asked in Luke 12:51 - "Do you suppose that I came to give peace on earth?" Many religious folks are under the delusion that peace is the ultimate goal. They believe that peace should be achieved at any price religiously. According to Jesus, that is simply not true. Our Lord freely acknowledged that He came to bring "division" and explained that His followers should expect division even among their immediate family members because of the gospel.
Jesus wanted His disciples then (and today) to know that faithfulness to Him is not about convenience or compromise but about conviction and courage. To be a genuine disciple of Jesus may very well result in discord in a family that was once united. How could this happen? When one member of a household genuinely accepts the truth, obedience to the Lord in all contexts becomes his priority. If others in the household reject the gospel or attempt to pervert it, conflict and opposition will inevitably arise (e.g., Mark 3:21,31-35).
Let it be noted that it is not true that the disciples of Jesus instigate this division. They should strive for peace as much as is possible without disobeying their Master (cf. Rom. 12:18). However, when one stands for the truth against evil and error that is in the kingdom and without, he will be hated and persecuted, often even by his own relatives (cf. John 15:18-20; II Tim. 3:12; Matt. 10:35-37). Such is the price all faithful stewards must be willing to pay (cf. Luke 14:26). Are you willing to pay the price or would you rather be a pleaser of men (Gal. 1:10)?