-Removed-It's all laid out in Revelation, you know, for those with ears to hear.
"Joining with Satan" means worshipping the Beast or his image or taking his mark. Surely you're aware of the concept of the AntiChrist? This is the Beast. The "mark of the Beast" is his mark that he or his agents place on either the hand or the forehead, to signify that you belong to him. Surely you've heard of this.
Revelation is different from all other books of the Bible, in that it has yet to happen, it is about a future time. Yes it was written long ago and the descriptions of things have to be filtered through the thinking of John at that time. This doesn't make the threat any less real.
-Removed-I'll try explain.
There is this, you either believe in God, or you don't. Irrespective of what you believe, IF you are saved you will go to heaven, if not, the only other option left is eternal suffering. There is no other island or another option you can escape too.
In life we are faced sometimes with many options, in some cases few options. Its like in some countries if you're caught stealing they will actually cut off your hand. You know the risk, so, do you want to keep your hand or loose it.
You can also decide if you want to be unfaithful to your partner, knowing that if you're caught out that you will loose him/her forever.
You can also not learn for the final exam, or drive a car without a valid license or pick a fight in the bar with the biggest and strongest guy - these are all choices, and we more than likely know how things will turn out in that you'd fail the exam, get a fine and get beaten to death I'm sure. We all know better.
The same goes with God - the choices are there, you know the risks, you take it or you leave it.