The Sermon on the Mount
Jesus' ministry consisted mainly of teaching, and his most famous collection of teachings is the sermon on the mount.
Jesus begins with the beatitudes, which delineate the characteristics of discipleship, then continues with a series of teachings that demonstrate how the kind of behaviour God expects must exceed that which Moses' laws demanded.
The heart of the sermon is the Lord's Prayer. What follows spells out the higher principles by which Christians must live. Humans may concern themselves with material things or spiritual truths, but when these two pursuits come into conflict, one must know which is the higher good.
Christians need not be anxious about things that God is already looking after. Jesus does not say that we should not concern ourselves with sowing or harvesting, but that we should not be unduly obsessed by it. Similarly, we should not worry about our clothing. He contrasts human anxiety with the "each day at a time" attitude displayed by the birds of the air and the complete inactivity of the wild flowers. Yet both of these are cared for by God.