If we accept there's a spiritual war on between God and the devil, the coming of Christ threw the whole battle into sharp relief.
From His advent on, Christians have been persecuted for their faith, starting with Christ Himself, who was crucified. When He was just two years old, His parents had to flee when Herod ordered the slaughter of the innocents.
Even the Mafia would probably draw the line at sending a bunch of hit men after a two year old child and his peers.
Within families, as Christ made clear, when it came to Him, father would be against son, mother against daughter etc. It didn't take long for the devil to use the Roman Emperors against the Christians, and the persecution continues to this day.
As for setting the earth on fire, I heard a homily last night (we had exactly this reading this weekend as part of the mass) in which it was claimed Christ wanted to set the earth on fire with His love.
Perhaps the speaker was correct, although I'm inclined to take a more calamitous view. I think that He foresaw the final unleashing of evil that would take place towards the end of time, and His advent would be the seed that would, in time, set the world on fire.
But then maybe we both have an element of the truth in our opinions.