Beniguman,
1.) You said,
“There are two problems with this position. First, as you yourself hint at, it is extremely difficult, if not impossible, to determine what will impose the most suffereing over the long term. ISIS might argue that they are causing a lot of suffering in the short term but in the long term the world will be vastly better off under a thousand-year world-wide caliphate. There is no way to demontrate, or even provide evidence, that ISIS is incorrect.”
They can “argue” that all they want. They would however need proof to justify it in order for anyone to take their arguments seriously: They would have to provide proof that a caliphate will cause the world to contain less suffering, and that there is no other way of accomplishing whatever the goals of having a caliphate would accomplish to lessen the suffering by causing less pain. If they did this then we could have a discussion.
2.) You said
“Second, and more fundamentally, by what objective means did you determine that “suffering” or the lack thereof, is the most important thing? Maybe truth is the most important? Maybe justice is the most important? My what objective means did you determine that it is moral and good to avoid suffering and immoral and bad to cause suffering?”
Pointless suffering by definition is something that is not what human beings (or even animals) consider a good thing, completely independent of any religious beliefs they may or may not have.
Objectivity will always from the standpoint of our existence even from a religious standpoint. What is “objectively” moral about pleasing a God? maybe God being constantly upset is more moral?