Some background may be in order. Callista is intended to be a historical romance portraying Christianity in the third century, specifically for a Catholic audience. The major characters of the work are Agellius and Callista herself. Curiously, while she is mentioned in conversation earlier, Callista does not appear in person until Chapter 10. Of the other characters, the only ones of importance are Juba and Jucundus, relatives of Agellius, and the priest Caecilius, who turns out to be a rather important historical personage, better known under one of his other names. The events take place in and around the town of Sicca Veneria (modern-day Al-Kaf or El Kef in Tunisia); while not nearly as large as Carthage to the northeast, it nonetheless is fairly important as the seat of the Proconsular Africa. The novel opens during a time of relative peace; Christianity has been declining in the area, to such an extent that Sicca has neither priest nor bishop, and the general view among the pagans in the area is that it is (finally) dying out there, although there is worry about the pervasive influence of Christians across the empire. There are, in fact, only a handful of Christians in Sicca at all, and most of those are merely nominal. While Agellius sincerely believes in Christian doctrine, his brother Juba is only nominally Christian (and that only when he feels like it, being inclined to local superstitions), and his uncle Jucundus is a pagan. Even Agellius is merely a catechumen, and has been for most of his life, not moving forward; he is "stuck fast in the door of the Church," and it's the view of both his brother and his uncle think it likely that only a little nudge will back him out. The particular nudge they think most likely to nudge him out is Callista, a beautiful and intelligent pagan Greek with whom he is smitten. Callista, however, turns out to be a more complex person than they had thought, for while she is definitely pagan, she has considerable sympathy toward Christianity. Things become complicated as the Decian persecution finally reaches Proconsular Africa and is officially put into effect there. And that's the basic line of the story.
Much of the novel is concerned with the conversion of Callista, and it is here that we see Newman's theology clearly manifested. Callista's conversion is, in essence, an interaction with three Christians: Chione, her maidservant, who has already died, and is the reason for Callista's sympathy toward Christianity; Agellius; and Caecilius, a priest who is riding around the countryside giving aid to Christians while hiding from the authorities, whose true identity we only learn in Chapter 20. In the Oxford University Sermons Newman has an important but often-overlooked sermon on personal influence as the means of propagating truth. In it Newman argues that moral truth, and in particular the truth of Christianity, is not generally propagated by miracles, arguments, or a Church hierarchy, although these may play a role in scattered cases. The real means by which moral truth is propagated in the world is personal influence, in the 'inherent moral power' we observe in some of those from whom we learn. These people are simultaneously the teachers of moral truth and the models by which we understand it. People in the world are drawn to the beauty and majesty of their characters; they recognize them to be rare and therefore precious; they regard them as in some sense out of their league; they are directly influenced by it. This is precisely the way Callista is affected by Chione, Agellius, and Caecilius; she feels that they are somehow sublime, that there is something in them which is worth having, even though she does not know quite what it is. Indeed, for a very long time she knows nothing of Christianity except that there is something attractive in Chione, Agellius, and Caecilius. As the narrator expresses it (in Chapter 27):
But then again, if she had been asked, what was Christianity, she would have been puzzled to give an answer. She would have been able to mention some particular truths which it taught, but neither to give them their definite and distinct shape, nor to describe the mode in which they were realised. She would have said, "I believe what has been told me, as from heaven, by Chione, Agellius, and Cæcilius:" and it was clear she could say nothing else. What the three told her in common and in concord was at once the measure of her creed and the ground of her acceptance of it. It was that wonderful unity of sentiment and belief in persons so dissimilar from each other, so distinct in their circumstances, so independent in their testimony, which recommended to her the doctrine which they were so unanimous in teaching.
In a slave, a country boy, and a learned priest she saw something that they all shared, particularly when they spoke of divine love. She has no commitment to Christian doctrines; but the Christian image found in her three sources, however vaguely defined, does provide a conduit by which those doctrines reach her in at least a vague form and impress her. The attraction borders on worship; as the narrator says of her attitude to Caecilius, "In spite of what she had injuriously said to him, she really felt drawn to worship him, as if he were the shrine and the home of that Presence to which he bore such solemn witness."
In Chapter 28 we are introduced to another way in which Callista's process of conversion exhibits echoes of Newman's theology, because Callista recognizes a sort of divine vocation. This is briefly mentioned in the sermon on personal influence to which I've just referred, when Newman talks about those "who acknowledge the voice of God speaking within them, and urging them heaven-ward"; but it is more extensively discussed elsewhere. The most famous discussion is that in the essay on Assent. In the Oxford University Sermons, the key passage is found in the sermon on the influences of natural and revealed religion. By 'natural religion' he means not religion based on reason alone, but those admirable aspects of the attempts by non-Christian people to worship God as they should. The foundation of this natural religion is conscience. Callista explicitly affirms natural religion in this sense:
"Well," she said, "I feel that God within my heart. I feel myself in His presence. He says to me, 'Do this: don't do that.' You may tell me that this dictate is a mere law of my nature, as is to joy or to grieve. I cannot understand this. No, it is the echo of a person speaking to me. Nothing shall persuade me that it does not ultimately proceed from a person external to me. It carries with it its proof of its divine origin. My nature feels towards it as towards a person. When I obey it, I feel a satisfaction; when I disobey, a soreness—just like that which I feel in pleasing or offending some revered friend. So you see, Polemo, I believe in what is more than a mere 'something.' I believe in what is more real to me than sun, moon, stars, and the fair earth, and the voice of friends. You will say, Who is He? Has He ever told you anything about Himself? Alas! no!—the more's the pity! But I will not give up what I have, because I have not more. An echo implies a voice; a voice a speaker. That speaker I love and I fear."
However, Newman holds that there is a weak point in natural religion understood this way; namely, that while it teaches "the infinite power and majesty, the wisdom and goodness, the presence, the moral governance, and, in one sense, the unity of the Deity," it nonetheless is limited in what it can convey of the divine Personality. He later did not like this way of stating it, since obviously many theists who are neither Jewish nor Christian believe in a personal God in some way or another. But something at least analogous to this lack Callista clearly feels, because she goes on immediately to say,
"O that I could find Him!" she exclaimed, passionately. "On the right hand and on the left I grope, but touch Him not. Why dost Thou fight against me?—why dost Thou scare and perplex me, O First and Only Fair? I have Thee not, and I need Thee."
Finally, in Chapter 29, Callista begins reading the Gospel of Luke and finally recognizes in clear outline the original Image which she had found echoed in Chione, Agellius, and Caecilius:
Here was that to which her intellect tended, though that intellect could not frame it. It could approve and acknowledge, when set before it, what it could not originate. Here was He who spoke to her in her conscience; whose Voice she heard, whose Person she was seeking for. Here was He who kindled a warmth on the cheek of both Chione and Agellius. That image sank deep into her; she felt it to be a reality.
This passage is suggestive of the passage on Gibbon in the later Essay in Aid of a Grammar of Assent, in which he argues that an adequate account of the spread of Christianity has to include the Image of Christ, which was spread by personal influence in preaching and teaching, and which made it possible for people to assent to Christian doctrine with a 'real apprehension'. This is, in fact, the form of Callista's own conversion:
She now began to understand that strange, unearthly composure, which had struck her in Chione, Agellius, and Cæcilius; she understood that they were detached from the world, not because they had not the possession, nor the natural love of its gifts, but because they possessed a higher blessing already, which they loved above everything else. Thus, by degrees, Callista came to walk by a new philosophy; and had ideas, and principles, and a sense of relations and aims, and a susceptibility of arguments, to which before she was an utter stranger. Life and death, action and suffering, fortunes and abilities, all had now a new meaning and application. As the skies speak differently to the philosopher and the peasant, as a book of poems to the imaginative and to the cold and narrow intellect, so now she saw her being, her history, her present condition, her future, in a new light, which no one else could share with her. But the ruling sovereign thought of the whole was He, who exemplified all this wonderful philosophy in Himself.
Callista, then, is a novel shot through with Newman's theology. I have only considered it insofar as it relates to Callista's conversion. There are other examples that could be drawn on; for instance, part of Caecilius's striking discourse on love and omnipotence in Chapter 19 could be fruitfully compared to Newman's sermon on the omnipotence of God as the reason for faith and hope. But it suffices to show, I think, how much Callista is really an idea-novel, which probably explains some of Newman's curious choices in its composition (e.g., the two main characters, the fact that the primary character does not appear until a third of the way into the book, etc.).