There are two aspects to the covenant God made with Abram: the promise of descendants and the promise of the land. Abram's response to the first is faith. But to the promise of the land he asks, "How am I to know?" The confirmation comes in the covenant ceremony which follows. This ritual of cutting the animals in two, where the parties walked between them, is what bound them in covenant. The splitting of the animal was a graphic way of showing what would happen if either participant failed to observe the terms. God, as fire, passes through, binding Himself to His promise. The land, however, will not come to Abram, but to his descendants.
Fast forward to the Gospel scene. Jesus (Abram's descendant) has just foretold His passion. His apostles, shaken, need a sign to reassure them. So Jesus takes Peter, James and John who, if we use our imagination, witness a similar covenant ceremony. The Old Covenant (divided) consists of the Law and Prophets, represented by Moses and Elijah. The three apostles are also "divided" by their commitment to Judaism and their following of Christ. Jesus is like the birds who are not divided. The Church Fathers see the turtledove as representing chastity (pure, unstained, integral) and the pigeon as simplicity (guileless, uncompounded). Jesus speaks with Moses and Elijah of His "passing" into the new land, His kingdom. Then the cloud covers them, and God "walks" between them with Jesus, proclaiming Him as His Son, ratifying the New Covenant which will come about through the passion, death and resurrection of Jesus. For a fleeting moment, the apostles are allowed to step into this "promised land". Peter, of course, wants to stay and build tents! But this was not to be. Fortified by this glimpse of glory, they would accompany Jesus on His journey for our salvation.
When the vision was gone, there was "only Jesus". What more could we want?! What more do we need? To be united to Him is to enter the Promised Land.