Musically Jesus was an only son is rather dull. Actually, it is remarkably dull, and it tends to have a critical effect on the flow of the album as a whole. For the first six tracks are musically dynamic and interesting. Then comes the less convincing Silver Palomino, and right after that the weary task of Jesus was an only son. And with only the too short relief of Leah before the too long The Hitter, the listener may really begin to long for the E Street Band. Oddly enough, a situation quite similar to what happens halfway through The Ghost of Tom Joad.
So the question arises, why Springsteen had to put Jesus was an only son on his new album. And the obvious place to look for an answer seems to be in the lyrics: The song must express something that he badly needed to express. But then again, you are baffled by the dullness, this time of the lyrics. For it doesn't feel very important to hear about Jesus walking around on different locations in the ancient Palestine, enjoying some pretty predictable conversations with his mother and his heavenly father. This may work in the Sunday school, but the question still stands, why Springsteen has felt it necessary to include it on his album. There seems to be no other possible place to look for an answer than the very last sentence of Jesus was an only son. On the other hand, this sentence may throw a lot of light on the meaning of the album as a whole.
As elaborated elsewhere, there is a strong bend toward a dualistic worldview on Devils & Dust: The human soul is represented almost as a cosmic battlefield where the light of God is fighting the darkness of more material forces. Now, in the history of Christian metaphysics the movement usually associated with such a view is Gnosticism, a way of thinking dating back to the very first centuries after Christ. The Gnostics thought of God as a distant saviour without any part in the troubles of mans physical life. Therefore, they didn't have to waste too many efforts on the well known problem of excusing God for the evilness of this world (the so called "theodice problem"). They simply placed the responsibility upon another fundamental principle - the matter - which was radically opposed to God. The co-existence of God and the matter as equally fundamental metaphysical principles makes this conception dualistic. The same thing will happen if you substitute "dust" or "devils" for matter.
Now, Gnosticism didn't become a part of the Christian tradition. On the contrary, it was fiercely attacked by the founding fathers of the Church - e. g. by Augustine in the 5th century. Some of the reasons for this hostility towards Gnosticism are quite simple. For it is clearly incompatible with the doctrine of God's omnipotence that he should be seriously opposed by any other power in the universe. And it is also a fundamental dogma of the Church that God is the creator of the world. But as the willed product of a good and almighty God the world cannot be considered as an absolutely bad thing. For all its shortcomings, it remains the work of God. It can hardly be entirely "God forsaken", such as the narrator tells us in Long Time Commin'.
It may sound somewhat ridiculous that a rock singer should worry about such theological matters. But there is little doubt that Springsteen has been approaching a Christian viewpoint for many years now - and is it so unreasonable to assume that he is trying to do it properly? As stated above, the reason for the inclusion of Jesus was an only son on the new album is most likely to be found in the last sentence. And this is where Jesus is facing his own death and tries to comfort his mother - the words that Springsteen lets him say are: "Mother, still your tears. For remember that the soul of the universe willed a world and it appeared".
The meaning of this utterance is profoundly anti-gnostic: There is only one "soul" of the universe, and because it is his creation you have to come to some kind of acceptance of the world as it is. It seems like Springsteen is here reminding himself about the Christian standpoint and tries to avoid a dualistic conception of two forces fighting for the mastery in mens lives. Fear might be "a powerful thing" but in the end there is only one almighty power.
The message of Jesus was an only son appears to be important for Springsteen because it counterbalances the tendency of most of the other songs on Devils & Dust. Taken as a whole, the album really is building up a dualistic perspective, where the material world is "stripped to its bones" and God seems to be desperately far away. Jesus was an only son contradicts this tendency. So the old controversy between Christianity and Gnosticism seems to continue on Devils & Dust.