The world is comprised of people with different philosophies. These philosophies include different religions (or lack thereof), different ethnic backgrounds, different traditions, rituals, educational systems, etc. These differences are part of what make life interesting. Scientology is just one more of these.
The important question in life is not whether we agree with these philosophies, religions, traditions, etc., but how we deal with those who practice them. If we can accept that they are simply different and treat them in a friendly manner, then we're probably doing ok. If we revile them because their beliefs are different, then we may need to examine our own beliefs.
It is true that some philosophies are, unfortunately, not compatible with society. For example, a belief system that includes human sacrifice is going to run afoul of virtually every legal system in the world, and as such, the practicioners will need to be dealt with by the legal system. However we, as individuals, probably would be best served by either a cordial relationship or no relationship with such people (we would, of course, be expected to report such a thing to the authorities as good citizens). If we take the next step and antagonize or attack (either in word or deed) these practicioners, we might find that we suffer as a result (as well as possibly legitimizing their belief).
If we know that a philosophy is dangerous, there are mechanisms in place for it to be dealt with. Short of that, stating negative things about a different belief is simply begging for the same kind of mudslinging at our own beliefs.
As to Fierce Diety's statement about missionaries, understand that many people are well served by missionaries from many different faiths. Also most faiths have a desire to spread their beliefs and missionaries are the way that this is done. You stated you are Catholic, so you should know that Jesus commanded all of His followers to spread the good news of salvation, in effect making every Christian a missionary.
-- Jeff
ps As a Chrisitan, I disagree with Scientology, and no, there isn't any way that anyone could get me to believe it, but I'll defend the right of people to believe it, and I'll act as a missionary towards them to tell them about Christianity.
