Originally posted by Tygur

I don't think it's really a scare tactic. A scare tactic like that would be bad, because it would scare people away from getting baptized.

I believe it is actually for the protection of the rest of the congregation. Remember that I said that it was the attitude of the person that determines whether he/she will be reproved or disfellowshipped. So if the person has a bad attitude and mingles in with the rest of the congregation, it could spread. Remember 1 Corinthians 15:33, which I quoted earlier. "Do not be misled. Bad associations spoil useful habits."

The "grave sin" is breaking off a promise made to God. Baptism is sort of an unwritten contract between you and God, saying that you will always follow him. Break it, and bad things happen, just like with any written legal contract.
The only thing I can conclude is that these are brainwashing tactics : you do as we (or God) tells you and you'll be in heaven, doubt us or even do as much as giving one note of critique and you're a bad influence. Some people will come and talk to you to change your mind (read : brainwash again). If all else fails you're branded a sinner.

Good to see you advocate common sense there