Let’s start by considering our humanity vis-à-vis the currently known universe. The great hymn, How Great Thou Art, includes the stars as evidence of God’s “power throughout the universe displayed.” And what person doesn’t feel like the psalmist:
When I look at your heavens, the work of your fingers,
the moon and the stars that you have established;
what are human beings that you are mindful of them,
mortals that you care for them? (Psalm 8:3-4)
This famous photograph by the Hubble telescope also speaks volumes (nearly 10,000 galaxies in the deepest visible-light image of the cosmos):
Stacked against the number of candidate exoplanets and speculation (however thinly supported) about life around other stars, the concept of anyone being God’s only son seems fraught from multiple angles. In this context, Jesus being a (terrestrial) human who is also God incarnate is a sweeping concept when juxtaposed against the vastness of the universe. Humans viewing themselves in this way in and of themselves would manifest an unjustified arrogance—accordingly that Jesus as mortal man displayed genuine humility before God demonstrates profound wisdom.
It may be difficult to imagine any human (including or especially Jesus) being an only son of God, when God’s purview encompasses the entire universe (in time, space, and any other dimensions we may not be aware of). Consider the first few sentences of the Gospel of John:
In the beginning was the Word, and the Word was with God, and the Word was God. He was in the beginning with God. All things came into being through him, and without him not one thing came into being. What has come into being in him was life, and the life was the light of all people. (John 1:1-4)
If without Jesus not one thing came into being, then by that definition Jesus also created all of those galaxies in the Hubble photograph as well as everything else not in the photograph. You may have heard what-if discussions speculating on how Jesus’ human life might apply to intelligent (but mortal) extraterrestrial beings—the significance of being the only son of God is that it would imply there is only one (human) Jesus, anywhere in the universe. The Apostle Paul goes further: For ‘God has put all things in subjection under his feet.’ (1 Corinthians 15.27)
In fairness to the biblical writers, they were not blessed with our eighteenth century Enlightenment understanding of the expanded universe, as they were all pre-Copernicus, Messier, Galileo, Kepler, et al. As science continues to discover increasingly more of the physical universe, claiming a commanding relevance of humans over and above this rich vastness takes on an increasing distasteful arrogance. What are human beings that God is mindful of us, indeed!
Even if there really are no other sentient creatures anywhere in the whole universe other than on Earth, there still are the billions of humans known to exist now and in the past. Speculation about extraterrestrial life is thus irrelevant regarding this known world that John 3:16 says God so loved. However, that still does not address the question of the mortal man Jesus being God’s only son.
The man Jesus must have been mortal like us, which is an essential pillar of orthodox theology—otherwise there could be no (full clinical) death on the cross, a necessary precursor to resurrection (and the redemption it establishes). “The Holy Spirit will come upon you,” [Mary] (Luke 1:35) doesn’t help for this discussion, as Jesus is still necessarily also a (terrestrial) human. Is the pale blue dot we live on really that significant to God over and above all the other potentially comparable dots throughout the universe?
None of this prevents Jesus (or any of the rest of us) from being unique in the universe. Does this uniqueness prevent any hypothetical extraterrestrial civilizations from being granted their own messianic saviors? A related and more important question is: Should we care? I think the answer is no—for both of the questions above.
Up to this point the “only” son problem is still not solved. Here is a solution that works for me: Jesus’ status as only son refers to his new birth at resurrection. Thus for now (until the general human resurrection), Jesus is at this point the only son (as the only one who is currently resurrected)—i.e. the first fruits (1 Corinthians 15.23).
For further context, consider the viewpoint expressed in the Letter to the Hebrews:
So also Christ did not glorify himself in becoming a high priest, but was appointed by the one who said to him,
‘You are my Son,
today I have begotten you’; (Hebrews 5.5)
For reference, the term “begotten” is derived from the literal meaning of “sired”—in this case, both spiritually and physically into a resurrected state. That the gospel narrative has Jesus’ resurrection at a specific point in time (Circa 33 C.E.), does not controvert his eternal pre-existence asserted at the start of the Gospel of John. (The Arian Heresy indeed belongs firmly in the dustbin of history).
The “only son” aspect has understandably sometimes been a stumbling block for interfaith efforts. Persons of faith in other traditions have views of God that are different from New Testament orthodoxy. The reality is that all of the traditional orthodoxies reject at least some of the beliefs of other faiths—otherwise they all would be the same faith, at least theologically. It is thus encouraging when people from different faith communities reach out to each other in prayerful mutual respect. For those of New Testament orthodoxy, Jesus’ command to Peter, regarding the disciple whom Jesus loved, is worthy counsel:
Jesus said to him, ‘If it is my will that he remain until I come, what is that to you? Follow me!’ (John 21:22)
The discussion of “resurrection” and “spiritual” topics is clearly outside of eighteenth century Enlightenment philosophy, and science in general. However outside of science does not automatically equal invalid. Reflexive dismissal of spiritual matters without holistic consideration of human nature is itself worthy of dismissal—including for scientific reasons, in my opinion. I think in the end it comes down to what one wants to believe, and sometimes for something to be found it must first be sought.
Sooner or later (at least at the end of mortal life) we all go through the crack in the cosmic egg. If New Testament orthodoxy is God’s plan, we’ll be in good company there.