Mormons are not the only non-Christian cult. There are Jehovah's Witnesses who believe that Christianity died with the last of the apostles. They believe that Christianity was not resurrected until their founder, Charles Taze Russell, began organizing the Watchtower Society in the 1870s.
In their view the cross is a pagan symbol adopted by an apostate church and salvation is impossible apart from the Watchtower organization.
While the Witnesses on your doorstep consider themselves to be the only authentic expression of Christianity, the Society they serve compromises, confuses or outright contradicts essential Christian doctrine.
They teach their devotees that the Trinity is a freakish-looking three-headed god invented by Satan and that Jesus is merely "a" god. In Watchtower theology Jesus was created by God as the archangel Michael, that during His earthly sojourn He was merely human, and then after His crucifixion He recreated an immaterial spirit creature.
They also deny the physical resurrection of Jesus. In fact, according to Russell the body that hung on a "torture stake" either dissolved into gases or is preserved somewhere as a grand memorial of God's love.
Well, Watchtower adherents are often willing to do more for a lie than Christians are willing to do for the truth. These and a host of other doctrinal perversions keep Jehovah's Witnesses from rightly being considered Christian. We need to be able to use those deviations as springboards or opportunities for sharing the truth and love and grace that only come with a relationship with the real Jesus and the real Gospel, which is a Gospel, not by works, but by God's grace through faith in Jesus Christ alone. We are not saved by what we do. We are saved unto good works. The Gospel is free - however, it will cost us our life. We take up our cross and daily follow the Lord.