The answer is: Anger

Matthew 5:21-22 - In the Sermon on the Mount, Jesus teaches that harbouring anger towards a brother or sister is equivalent to committing murder in one’s heart. This radical redefinition of sin emphasizes inner attitudes over actions, illustrating that true righteousness involves not just the avoidance of sin but also cultivating pure intentions.

In the beautiful and challenging Sermon on the Mount, Jesus flips society's concept of righteousness on its head. It's intriguing to consider that when He talks about anger being as severe as murder, He’s delving deep into human emotions. Jesus draws a line between external actions and internal feelings, unveiling a revolutionary thought for His listeners: faith isn’t just about outward display but also about what's brewing inside.

The use of the term "brother" or "sister" shows that these teachings are deeply personal. It suggests a sense of community and integrity among individuals that’s just as important as maintaining one's personal moral compass. Gossip, hurtful thoughts, and grudges can sometimes weigh heavier than physical actions; the bitter seed of anger can grow into significant divisions and conflicts.

Did you know that the ancient Hebrew concept of the heart encompassed the entire essence of one’s being, not just emotions? The term for heart (levav) implied intellect, thoughts, and spiritual disposition. This speaks to Jesus’ point further—it's about transforming the ‘heart tools’ we have at our disposal.

As you reflect on this teaching, think of the broader context of love and compassion that threads through His entire message. It’s all about creating a deeper, loving relationship with each other and ourselves, a task that many of us are still navigating today. Recognizing these inner struggles can be the first step toward genuine reconciliation and peace.