The river of life and the water of life are symbols of eternal life. Remember in John 4 (the story of the Samaritan Woman) that the water of life will spring up from within. The tree of life was present in the Garden of Eden. Theologians consider it a great mercy that God banished Adam and Eve from the garden before they ate of the tree of life. To become eternal beings in their sinful and broken state would have been punishment, indeed. Here the tree of life feeds the city. The tree of life brings health and wellness to the residents of the city. All may partake of it freely and often. All of this symbolizes for us that in the new earth there is no pain, no disease, and we have life everlasting.
Revelation ends with an epilogue that contains three things: 1) The confirmation of the genuineness of the prophecy (22:6-7, 16, 18-19); the imminence of Jesus second coming (22:7, 12, 20); the warning against idolatry and the invitation to enter the city (22:11, 15, 17). Between the invitation and the conclusion is a final warning to take seriously what John has written and not to change the words of the book. In the end the book is a book of hope and anticipation. Times may seem dire and dark and difficult but justice will prevail, wrongs will be righted, broken will be healed and God's purpose will be accomplished in all of creation.
The book ends with a reminder that Jesus is coming and, finally with a blessing.
Tomorrow we will begin Paul's letter to the Galatians -- watch for introduction and chapter 1.