1. JWs must believe the teachings of the WT because the Governing Body claims it is the only one that can properly interpret the Bible. It claims to be the "faithful and discreet slave" of Matthew 24:45, appointed by Jesus to give his true followers spiritual "meat in due season."
2. The WT cites several passages against the deity of Christ. For example, in John 17:3 he himself calls the Father "the only true God." They cite the subordination passages, such as John 14:28, which show the Father's positional superiority over the Son. They note that Colossians 1:15 calls Jesus the "firstborn of all creation," which the WT says means that he was the first thing Jehovah created. They also note that Jesus died and ask, "If Jesus is God, who ran the universe for 3 days when he was dead."
As Vanderhoven said, this topic merits a thread of its own. In fact, entire books have been written on this topic.
3. Non-anointed JWs (the great crowd of other sheep) have no assurance of salvation since they believe their salvation depends on their faithfulness to Jehovah and his organization. Even after the end of the Millennial Kingdom and even if they pass the "final test" by Satan, if they sin willingly (as Adam did), Jehovah will annihilate them. So their salvation will never be 100% secure.
It's different for the anointed 144,000 after they die because they will go to heaven and receive spirit bodies, so they will no longer be subject to temptation. Until they die, though, they could still forfeit their salvation if they are unfaithful.