Finished reading the Gospel of Luke today. It's the longest of the gospels, and, I think, my favourite, because it emphasizes Jesus's mercy and includes the parables of the Prodigal Son and the Good Samaritan, which the others leave out. This book is filled with stories of Jesus's life and teachings, as well as an account of his birth and youth, his baptism by John the Baptist, and his ministry and crucifixion.
There is so much in each gospel, it's hard to know what and how to summarize. One of the most important elements, as far as I can tell, though, is that Jesus is often asked what mankind should do to live righteously. To attain eternal life, Jesus says the most important thing to do is:
"Love the Lord they God with all thy heart, and with all thy soul, and with all thy strength, and with all thy mind; and thy neighbour as thyself." -- Luke 10:27
This is obviously very important, because it is repeated more than once in this and the other gospels. Love God above all, and treat others as you would want them to treat you. So simple to understand, and yet so difficult to do.