The Book of Luke is one of four books that sets forth the four-fold earthly-life and death and resurrection of the Lord Jesus Christ. This book does not contain everything that Jesus said and/or did. It emphasizes the Lord Jesus Christ being God's man (human-being) and it details his interaction with other people according to God's will.
Luke had not been one of the twelve men with Jesus during his ministry prior to Jesus' death. He wrote this book after the resurrection and ascension of the Lord Jesus Christ into heaven and after the out-pouring of holy spirit on the day of Pentecost (refer to Acts 2). He also wrote the Book of the Acts (Practices) of the Apostles which overlaps the end of this "Gospel" and he is referred to in Colossians 4:14, II Timothy 4:11, and Philemon 1:24.
Luke wrote this book by revelation – he received the information of what to write via his holy spirit-life (not from his intellectual memory).
