Ok, as persons who have experience with the SWP, can you tell me if they actually genuinely believe they will achieve their aims through the methods they currently use?
Let's lop off the last bit, since if you asked some Social Workers they would almost certainly point out that methods change with circumstances, that their 'party' is very flexible in changing its ways of doing things (which is true, sort of) and obviously circumstances are not going to remain the same between now and the triumph of world proletarian socialist revolution under the tutelage of the Social Workers and their foreign subsidiaries in the IST. (The IST is the Social Workers' Micro-International, from which people who don't accept the line from London are expelled amid much acrimony).
Your question then becomes: Do the members of this particular Church of Latter Day Bolsheviks really believe they can succeed?
Many do, of course - just as many Jehovah's Witnesses believe in the Second Coming. Others, IME, once you get to know them, don't really or aren't sure. Some take the view that it's gotta be worth a try. (A great many give up trying and leave. The Social Workers are notorious for their high turnover of members.)
I'm not sure whether rival sects, like the SP, are different in that respect. My guess is that they are not.
People in sects do often retain some ability to spout the line long after they have lost the faith, though.