What does Mark 12:40 mean?

A scribe is an expert in the Mosaic law, like a lawyer. They do not receive a commission for their teaching, and in fact, the Tractate Nedarim 37a, 62a states that it is unethical to use good deeds or the Torah to "magnify thyself" or as "a spade to dig with." The scribes have found a way to work around this prohibition.

Instead of using the Torah to earn a salary, the scribes encourage the belief that the people will be blessed by God if they support the scribes financially and practically. The rich can afford to be generous, but widows are often more tender-hearted than financially astute. The scribes either take advantage of that generosity or, for those widows who have asked scribes to manage their assets, steal from them. Either tact is foolish considering God has said He will judge those who prey on widows (Isaiah 10:1–4).

Paul doesn't subscribe to scribal tradition. He teaches that religious teachers should be directly supported by those they serve (1 Timothy 5:17–18). Many self-titled preachers today combine the two sentiments, insisting that if their congregation gives them enough money for a lavish lifestyle, God will bless the givers. Like the situation with the scribes, this is false teaching and spiritual abuse.

Jesus isn't condemning long prayers, as we are told to "pray without ceasing" (1 Thessalonians 5:17). He is saying we shouldn't pray grandly as if we are sincerely talking to God when we're really looking for attention. "Pretense" is from the Greek root word prophasis, which means "for show, with a false motive." One of the problems with praying for worldly reasons is that you only get worldly rewards (Matthew 6:5).

Pretentious prayers are still common today. Many people sermonize when they ought to be praying, instead. They may start with, "God, let this person understand…" and then go off on a misdirected lecture about everything they want the person to change. Or they may "humble-brag" by "thanking" God for a particularly long list of perceived blessings (Luke 18:9–14). Prayers of thanks, or expressions of self-esteem, are valid if they are sincere and done in secret (Matthew 6:6). When they are done in public, they serve as attempts to either scold or impress others.

Mark 12:38–40 is a quick synopsis of Jesus' condemnation of the scribes and Pharisees. At the end of Matthew's account of this incident in Matthew 23, Jesus calls the religious leaders vipers sentenced to hell and compares them to Cain, who killed his brother Abel out of jealousy for his relationship to God (Matthew 23:33, 35). Jesus then mourns for Jerusalem and its people who kill the prophets God sends to draw them to Him (Matthew 23:37–39).