See this passage explained in Matthew 16:1-12.
Sighed deeply in his spirit - His heart was deeply affected at their wickedness and hypocrisy. The word "spirit" here is taken as the seat of the emotions, passions, affections. He drew groans deeply from his breast.
No sign be given - That is, no such sign as they asked, to wit, a sign "from heaven." He said a sign should be given, the same as was furnished by Jonas, Matthew 16:4. But this was not what they "asked," nor would it be given "because" they asked it.
Beware of the leaven of the Pharisees - See Matthew 16:6.
Of Herod - Of the Herodians - of Herod and his followers. Matthew, instead of "Herod," has "the Sadducees." It is not improbably that he cautioned them against them all. The Pharisees sought his life, and were exceedingly corrupt in their doctrine and practice; the Sadducees denied some of the essential doctrines of religion, and the Herodians probably were distinguished for irreligion, sensuality, and corrupt living. They were united, therefore, with the Pharisees and Sadducees in opposing the claims of Jesus. Matthew has recorded his caution to avoid the Pharisees and Sadducees, and Mark has added, what Matthew had omitted. the caution likewise to beware of the Herodians. Thus, the evangelists speak the same thing.
Other Barnes' Notes entries containing Mark 8:20:
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