Constantine is a spiritual thriller that features Keanu Reeves, most known from movies like Speed and the Matrix trilogy, playing the title character John Constantine. Constantine, a chain smoking cynic, has a special gift of being able to see angels, demons, and half-breeds.
The movie opens with the discovery of the "spear of destiny" which, according to Catholics, had been the true cause of the death of Jesus. The whole movie is based on the concept that Satan's son Mammon wants to have his own hell on earth.
The half-breed angel Gabriel wants humans to be worthy of God's love (by proving themselves truly good beings while surviving hell on earth,because usually if a man repents truly for his sins shall be forgiven by God and this is not favored by Gabriel who thinks that its unfair for the humans to attain God's love so easily) and hence tries to assist Mammon to come to Earth, since he needs divine assistance to do so. Now it's up to John Constantine to save the day.