Were you wondering about The Everlasting Man?

I overheard that someone was trying to learn more about GKC's masterwork, The Everlasting Man, specifically why the book has that title.

It is simple GKC's wonderful title for our Lord, Jesus Christ, and can be derived from the twin titles of the twin halves of the book: the first half is called "On the Creature Called Man" and the second half is called "On the Man Called Christ".

But this may not be enough for you. There is one place that I know of in Chesterton's writing where he uses the phrase as a title of a person, and not of a book, and there is only one possible solution to Whom the title refers to. Read it for yourself:

...the doctrine of the Dual Nature of Christ is in the most genuine sense interesting; it ought to be interesting to anybody who can understand it, long before he can believe it. It has what can be called with all reverence a stereoscopic interest; the interest of having the two eyes in the head that create an object; of having the two angles in the triangle that determine the third. The old Monophysite sect declared that Christ had only the one divine nature. The new Monophysite sect declares that He has only the one human nature. But it is not a pun or a trick but a truth to say that the Monophysite is by nature monotonous. In either of his two forms, he is naturally on one note. The question of objective historical truth is another question, which I am not arguing here, though I am ready to argue it anywhere. I am talking about intellectual stimulation and the starting point of thought and imagination. And these, like all living things, breed from the conjunction of two, and not from one alone. Thus I read, with sympathy but a sympathy that hardly goes beyond sentiment, the studies of the modern Monophysites in the life of the limited and merely mortal Jesus of Nazareth. I respect their respect; I admire their admiration; I know that all they say about human greatness or religious genius is true as far as it goes. But it goes along one line; and cannot convince like the things that can converge. And then, after reading such a tribute to an ethical teacher in the manner of the Essenes, perhaps I turn another page of the same or some similar book; and come upon some phrase used about a real though a pagan religion; perhaps some supposed parallel of what is called a Pagan Christ. I find it said, if only of Atys or Adonis, "There was a conception that the god sacrificed himself to himself." The man who can read those words without a thrill is dead.

The thrill is deeper for us, of course, because it is concerned with a fact and not a fancy. In that sense we do not admit that there is any such parallel with the legends of the ancient pagans as is implied in the books of the modern pagans. And indeed we are surely entitled to call it mere common sense to say that there can be no complete parallel between what was admittedly a myth or mystery and what was admittedly a man. But the point here is that the truth hidden even in myths and mysteries is altogether lost if we are confined to the consideration of a man. In this sense there is an ironic and unconscious truth in the words of the modern pagan, who sang that "the heathen outface and outlive us," and that "our lives and our longings are twain." It is true of the modernists, but it is not true of us, who find simultaneously the realization of a longing and the record of a life. It is perfectly true that there were in many pagan myths the faint foreshadowing of the Christian mysteries; though even in saying so we admit that the foreshadowings were shadows. But, when all imaginative kinship has been explored or allowed for, it is not true that mythology ever rose to the heights of theology. It is not true that a thought so bold or so subtle as this one ever crossed the mind that created the centaurs and the fauns. In the wildest and most gigantic of the primitive epic fancies, there is no conception so colossal as the being who is both Zeus and Prometheus.

But I only advert to it here, not as arguing its truth against those who do not believe it, but only as insisting on its intense and intellectual interest for those who do believe it. I only wish to explain to those who are worried in this way, that a mind filled with the true conception of this Duality has plenty to think about along those lines and has no need to dig up dead gods to discredit the Everlasting Man. There is no necessity for me to be a Modernist in my own thoughts, or Monophysite in my own thoughts; because I think these views much duller and more trivial than my own. In the beautiful words of the love-song in The Wallet of Kai Lung, one of the few truly psychological love-songs of the world: "This insignificant and universally despised person would unhesitatingly prefer his thoughts to theirs."
[GKC The Thing CW3:301-2, emphasis added]

And this even more insignificant person thinks that that is a sufficient answer.