If we don't believe in the message enough to tell others what it is, why should they believe it? The only thing I feel we should exercise caution in is what we tell people in addition to the message - that is where we get into problems. The basis of Christianity is that Christ died for our sins and if we repent and accept him we wash away those sins.
This problem is as old as the new testament, Paul chided peter(? I think it was him, late and I am tired!) for telling converts they had to be circumcised, and there were those trying to get people to observe jewish fasts and holy days - Paul preached 'tolerance' if you will on some things, the things that did not really matter. He was not trying to change the culture of the people but their hearts. But while non-jews were not bound by the old covenant they were still bound by the new one and by the example of Christ - who did believe in sin (which is where we get mired in detailing out what things are considered wrong in God's eyes).
There are problems to be examined but I think they boil down to an analysis of both the old and new testaments to get an overall picture of sins of the heart versus procedural sins of covenants. The whole problem with that though is people start interpreting things all sorts of ways to fit their needs. But if we teach the basics then in their heart they will be convicted by the holy spirit and know right from wrong - those who want to do 'wrong' are most often the ones trying to find a loophole in the bible to justify their deeds.
I would go on but it is late, sorry for the long rant!