We used to believe that the inspired books of the Bible were actually dictated by God. I have seen a church window that depicts St. Luke pausing, a writing quill in hand, and one ear cocked, waiting to hear what an angel was dictating to him.
St. Luke tells us it didn’t happen that way. At the beginning of his Gospel he wrote:
Many have undertaken to compile a narrative of the events that have been fulfilled among just as those who were eyewitnesses from the beginning and ministers to the word have handed them down to us. I too have decided, after investigating everything accurately anew to write in down in orderly sequence.
There he says that he gathered notes made by people who listened to Jesus, and he fitted those note together in continuous orderly sequence.
People who are familiar with the Gospels have noted that a third of the words and stories in Matthew’s Gospel appear in Luke’s Gospel. It is a good bet that they used the same eyewitness source. German scholars whose word for a source is Quell, shorten that, saying that Matthew and Luke got the passages they share at are same from Q. Both Matthew and Luke used what they got from Q in a way that got their special point of view across.
In today’s Gospel Luke wrote that Jesus “stood on a level ground,.”
St. Matthew, writing for Jews, and desiring to contrast our Lord’s teaching with what Moses taught from Mount Sinai pictured Jesus as giving his teaching from a mountain in the Sermon on the Mount. Luke, here, writing for democratic Greek people, pictured Jesus as teaching on the level.
Now, although Luke and Matthew used other sources, and although both of them used their own ideas, still their Gospels are inspired. God inspired them to use their own ideas and style in writing.