After experimenting for a while (God bless the poor victims' souls) and with the help of the Brady guide I have compiled a handy list related to the cult.
For every victim delivered to the cult, Trevor receives $1'000. After he delivered his fourth victim, he will be "invited" by the cult, resulting in a bloodbath shootout.
Because of that, you'll have to decide which victims to deliver if you plan on exterminating the cult.
Keep in mind that only Trevor can make deliveries to the cult.
Here's a list of potential victims, and the rewards they give, when you don't sell them out to the cult.
Burial:
There's a woman lying on the ground, surrounded by digging men. She's about to be buried alive. The creeps carry quite a bit of cash around. Saving her doesn't instantly give you a reward; her father - a gangster - will be very happy if you do save her and wire $60'000 to your bank account after a while.
Domestic:
There's a man who's just been thrown out of his home by his wife, who suspects him to be cheating on her. He asks you to drive him to the golf club. Do that, and he'll give you $80, his contact info, and will become available as a "Hard" opponent to play golf against.
Drunk Driver 1:
Two men are arguing. One of them asks you to drive the drunk other home. Back home, the drunkard will give you $80 and pass out.
Drunk Driver 2:
A young couple ask you to drive them to their motel. At some point they'll nonchalantly have sex in your car. They reward you with $80. Since they are two people, delivering them to the cult awards $2000.
Escape Paparazzi:
An actress wants to avoid Paparazzi, because she's not wearing enough make-up. She'll pay $750 for your troubles.
Getaway Driver:
Two robbers threatening a merchant at gunpoint ask you to help them get away. Do it, and they'll give you a $1000 cut. One of them turns out to be none other than Packie McReary from GTA IV, a fairly skilled Gunman for use in future heists.
Hitch Lift 2:
[Hitch Lift 1 is not a potential cult victim; this is not a typo]
A woman with red backpack asks you to drive her back home. The reward for doing so is her contact info.
Hitch Lift 3:
A woman is waiting for a car to stop and pick her up. She wants you to take her to a broadcasting station close by the Vinewood sign in order to surprise her boyfriend. The only reward you'll ever get, is her boyfriend threatening to punch your teeth out, if you don't knock him out first. Oh, you could gun him down too, but where's the fun in that?
Hitch Lift 4:
A bride wants to escape from her groom. Why she ever agreed to marry him is a mystery, even to her. There is no reward for helping her.
Snatched:
A woman is being abducted by the Lost. You'll have to kill them, then save her. She'll ask you to drop her off someplace, where a friend will pick her up. She won't give you any reward, despite having to deal with a hail of bullets, talk about ungrateful...
Best Answer
According to this page, finishing Mr. Philips should unlock the two next missions: Nervous Ron and Trevor Philips Industries, given respectively by Ron and Tao Cheng. So except if you're experiencing a bug blocking the following missions, you should already have access to these two.
If this is the exact point where you're stuck, maybe do you have a save file right before the first Trevor appearance? If you do, try to load it and replay the missions to see if something is triggered this time.