Matthew, who was also known as Levi (Matt. 9:9; Luke 5:27), was a tax collector who had left his business to follow Christ. He was one of the 12 Apostles and in characteristic fashion humbly referred to himself as a tax collector, or publican (Matt. 10:3). Many have noted that Mark is a book of action that does not stress the lengthy discourses that Jesus sometimes used in His teaching. Matthew, however, included several lengthy discourses in his book. He also repeatedly used the phrase "kingdom of heaven" as he presented Jesus as the Messiah-King.
The first of the discourses in Matthew is what we know as the Sermon on the Mount (chapters 5-7).
The Sermon describes the kind of righteousness Jesus expects of His followers. It must exceed the righteousness of the Pharisees (Matt. 5:20) and will be realized fully in Christ's Kingdom. It is also a standard we should strive for.