You are right but wrong. Yes without question Christ's death was sufficient to absolve Christians of their sin. BUT you cannot expect to escape the consequences of your sins. Jesus death on the cross saves us from the ETERNAL consequences of sin. It saves us from ETERNAL damnation. It does not erase the TEMPORAL consequences of our sins.
For example if a person takes drugs and robs banks he causes a tremendous amount of pain and loss to all those who are affected by his actions. He can repent and God will forgive him and he will go to heaven. But in some way he will have to make restitution for the hurt has caused other people. His earthly punishment will be jail time (no matter how sorry he is, and even though he is forgiven by Jesus death on the cross), he still has to pay the earthly consequences of his crimes. In that instance, his jail time IS Purgatory. If he robs the bank, repents and dies before he serves his jail time, then he will go to Purgatory and after he is purged by some suffering, he will go to heaven. Purgatory can be on earth, in the afterlife before entering into heaven OR BOTH.
As St. Paul says, "the person will be saved, but only as through fire. The doctrine makes perfect sense. Our sins have earthly consequences, bad ones. We have to pay them one way or another before we can go to heaven. That's justice. Jesus paid the eternal price for our sins. That's mercy.
You really don't expect to sin and not suffer in any way do you? Of course not. So why would the idea of Purgatory bother you?
"If he robs the bank, repents and dies before he serves his jail time, then he will go to Purgatory and after he is purged by some suffering, he will go to heaven. Purgatory can be on earth, in the afterlife before entering into heaven OR BOTH."
Indeed this is so.
When one considers the real horror of death by AIDS, one can see that many of these men have indeed paid for their sins. God does not mess around.