He called his followers to a higher standard of truthfulness, one in which a person's word could be depended on, without the necessity of an oath. Trust and integrity are the foundation stones of our relationships.
Relationships grow strong and deep when there is mutual trust.
Other people, especially those close to us, need to be able to depend on us and believe that what we say is what we mean and what we will do. Without trust, love can be stifled. If we begin to feel comfortable with our dishonesty, we can easily deceive ourselves as well as others. We may never see the patterns of sin in our lives that are blocking God's love and our growth in holiness. We can never deceive God, however, who is able to look into our hearts and know what lies there.