Q&A Jesus Makes the Unclean Clean

Why have we not had a service on Luke 6: 38? people say they can't afford to tithe 10%, but in actuality, they really can't afford not to... 

I don't know if Luke 6:38 has been preached at Gateway before (I haven't been here that long, but I do know that Pastor Ben has preached on giving and the tithe many times). When we preach through the gospel of Luke we'll be sure to preach this passage. 

Why does Jesus tell the demon and the leper to be quiet about what he has done? 

A very helpful (and short) video summary of why Jesus tells people to be quiet about what He's done can be found here. 

In Mark 1:43-45, Jesus tells the man who was healed not to tell anyone how he was healed. Why did Jesus not want the man to spread the news? Wouldn't the telling of the story help bring glory to God? 

See my answer above with the link to the video. 

Wow! Great sermon today, Josh! Was the John MacArthur quote from one of his books, and if so, which one?

Not from a book. From his sermon on this passage.