I've been keenly aware of heart conditions recently since Mom had a heart attack. Did you know the heart muscle can calcify? That was news to me. As a result of the hardening of the heart muscle, It can't pump life sustaining blood through the body as well as it should. This happens due to unhealthy eating habits, what someone takes in. There is another heart condition called cardiomyopathy, which the heart muscle has a thicker, stiffer wall. It has the same problem of pumping blood effectively. A person with this condition is born with it. Young athletes have dropped dead in the middle of playing a game. Both conditions are deadly.
In our passage, we see Jesus dealing with the Pharisees and Sadducees, who had a serious spiritual heart condition. They demanded a sign from heaven that He show them proof of who He is. There were already plenty of signs He was the Son of God at His baptism (Matthew 3:16-17), He had just finished feeding 4,000 people from the little bit of bread and food He and the disciples had with them. His works and His teachings in light of the prophecies should have been convincing enough. Jesus pointed out to them their blindness to the truth of the Word of God that they couldn't even see "the signs of the times" (verse 3). The only sign He said they would get would be the sign of Jonah, in other words, His rising from the dead in 3 days. Their hearts were hardened because of their pride, unbelief, and hypocrisy.
A serious, less obvious heart condition existed in the disciples.They had just been with Jesus when He fed the 4,000 with the remnants of the fish and bread they had with them. not only that, they had been with Jesus for quite some time during His ministry. they heard His teaching and witnessed His miracles. At this point, they realized they had forgotten to get more bread. What they had was gone. Somebody goofed! I can just hear the discussion, "Hey, Bro. I thought you were getting the bread."
"No, man. That was Your job!"
"Ugh! Way to go!"
Jesus warned them about the Yeast of the Pharisees' and in Mark 8, "of Herod". That was the reminder bringing this flurry of discussion.
Yeast was considered a symbol of sin or evil. The Pharisees had all the appearances of being righteous, but their hearts were hardened, a terminal spiritual disease. Jesus didn't want His disciples to suffer the same fate by taking in the bad food they dished up regularly- by following their teachings. He overheard the murmurs of the no bread fiasco. So He reminds them of the two miracles of feeding 5,000 then 4,000. If He could provide for 9,000 families, was the bread for his twelve man troupe too hard? These men had already been displaying a bit of heart problems. They lost sight of Whom they followed! Finally the light bulb moment happened after He told them it had nothing to do with bread. Jesus didn't want them to follow their bad teaching. Even a little yeast, evil, would ruin them. Just as a doctor would warn us to avoid certain food that causes heart disease, the Great Physician warned them to avoid spiritual heart disease.
Are we different from the disciples? I know that I forget sometimes what the Lord has done in the past when faced with a concern. A simple reminder is all it normally takes. Staying in the teachings of the Bible, reflecting on God's character and His activity in your life from the past, while being an active part of spiritually healthy believers will help you avoid spiritual heart disease.
May God bless you!