Explain the unforgivable sin of blasphemy of the Holy Spirit.
We'll be getting to this in a few weeks in the gospel of Mark.
In regards to the demons who were testifying that Christ was the son of God- why did he only cast them out and not destroy them?
I don't know if our expectation should be that demons will be destroyed. They will be judged for all eternity, but they won't cease to exist. But to (somewhat) answer your question. Jesus didn't send the demons to Hell at this point because their day of judgment had not yet come. But it is coming.
John 13 says Jesus washed the disciples feet and ate the Passover. Then in chapter 18 we find the Jewish leaders not going into the palace because they wanted to eat the Passover.
I don't understand.
A couple of things may be helpful in understanding what John is telling us. First, Jesus and His disciples ate the Passover meal on Thursday evening in John 13 (thus why we have our Maundy Thursday service). Chapters 13-18 (even chapter 19) all record what happens within a twenty-four hour period. And for Jews, the day ran from sundown the previous day (Thursday evening) through sundown the next day (Friday). So it's still technically the same day in John 18:28, when the Jews don't want to defile themselves (even though for us it would be a different day of the week).
Second, scholars also note how the word "Passover," at times, didn't explicitly mean the one meal, but the entire celebration of meals that took place during the Feast of Unleavened Bread. This is based on Luke 22:1 which states, "Now the Feast of Unleavened Bread drew near, which is called the Passover." The thinking here is that the Jewish leaders were not concerned so much with the one Passover meal (which they may have already celebrated), but being defiled and unable to continue in the entire Passover celebration (the Feast of Unleavened Bread).