Did Satan really not understand God's salvation plan? I've always been told that when Jesus died on the cross, he ultimately defeated Satan and won victory over death. Satan was not aware of the secret plan and thought that he had defeated Jesus until Jesus died and he realized what had really been going on.

I don't find that so hard to believe, but I've been thinking about it lately ... Since there are a lot of prophecies about the son of God in the Old Testament -- and Jesus fulfilled them all -- shouldn't it have been quite clear for the Devil that God's plan was to sacrifice his only son, namely Jesus, on the cross to cleanse all of our sins? I figure that he should have the necessary intelligence to come to that conclusion. Or is it something along the lines that he's not able to understand the ways of love and truth, since he hates love and truth and all that is good, and has nothing to do with any of it?

So, my question is: Was Satan really taken by surprise when Jesus died on the cross?
No I don't think he was surprised at all. But that doesn't mean he wasn't disappointed. I'm sure he held out a false hope that things might go his way.

Satan is not omnipotent and could not have known exactly what God was planning, but he would have known and been able to understand the prophecies as well or better than men and it was never any secret that he was going to be crushed. However we must remember how he got to be Satan in the first place. His pride blinded him to think that he could rise and challenge the living God. That kid of pride is debilitating to comprehension and it deludes one into thinking an outcome could be  different than the obvious.

I think this can stand as a warning to Christians because we have the same root of pride in our nature. If we willfully object to the sovereignty of God in our personal lives we will be blinded by this pride.