Mark is warning believers to remember that God’s Word is God’s Word, and nothing that deviates from or violates the Word of God is from God. To put the commandments of men, the teaching of infidels, or policies of political parties ahead of God’s Word exposes people for what they really are. They can claim to be devout Christians, but Jesus said they are hypocrites.
The holy Rabbi of Nazareth invited the unholy and ungodly tax collector to be His disciple. The disciples who were already with Jesus must have gasped, and the people in the crowd probably began to whisper in disbelief, but the most surprised person at that moment would have been Levi.
Once people realized how special Jesus was, everywhere Jesus went the people crowded around Him. Eventually some of the religious elites began to worry that Jesus might challenge their positions or their authority, so they also began to show up in order to critique and criticize Jesus. They were constantly watching with a critical eye and listening for something with which to accuse Jesus of, such as, fraud or blasphemy. Sadly, in process of their criticism, they likely deprived themselves of the ministry that could have been for them. People who come to church only to pick apart the sermon and criticize the preacher are rarely blessed.