It revolves around a soldier and noble called Marcus Vincicus and his love for a Christian girl called Lygia.
Here's Vincicus speaking to Lygia about his change of heart regarding her faith.
"Paul has convinced me, has converted me; and could it be otherwise? How was I not to believe that Christ came into the world, since he, who was His disciple, says so, and Paul, to who He appeared? How was I not to believe that He was God, since He rose from the dead? Others saw Him in the city and on the lake and on the mountain; people saw Him whose lips have not known a lie. I began to believe this the first time I heard Peter in Ostrianum, for I said to myself even then: In the whole world any other man might lie rather than this one who says, 'I saw.' But I feared thy religion. It seemed to me that thy religion would take thee from me. I thought that there was neither wisdom nor beauty nor happiness in it. But to-day, when I know it, what kind of man should I be were I not to wish truth to rule the world instead of falsehood, love instead of hatred, virtue instead of crime, faithfulness instead of unfaithfulness, mercy instead of vengeance? What sort of man would he be who would not choose and wish the same? But your religion teaches this. Others desire justice also; but thy religion is the only one which makes man's heart just, and besides makes it pure, like thine and Pomponia's, makes it faithful, like thine and Pomponia's. I should be blind were I not to see this. But if in addition Christ God has promised eternal life, and promised happiness as immeasurable as the all-might of God can give, what more can one wish? Were I to ask Seneca why he enjoins virtue, if wickedness brings more happiness, he would not be able to say anything sensible. But I know now that I ought to be virtuous, because virtue and love flow from Christ, and because, when death closes my eyes, I shall find life and happiness, I shall find myself and thee. Why not love and accept a religion which both speaks the truth and destroys death? Who would not prefer good to evil? I tough thy religion opposed happiness; meanwhile Paul has convinced me that not only does it not take away, but that it gives. All this hardly finds a place in my head; but I feel that it is true, for I have never been so happy....O Lygia! Reason declares this religion divine, and the best; the heart feels it, and who can resist two such forces?"
...After a while he said with a lowered and quivering voice: "Thou wilt be the soul of my soul, and the dearest in the world to me. Our hearts will beat together, we shall have one prayer of and one gratitude to Christ. O my dear! To live together, to honour together the sweet God, and to know that when death comes our eyes will be open again, as after a pleasant sleep, to a new light, - what better could be imagined? I only marvel that I did not understand this at first. And knowest thou what occurs to me now? That no one can resist this religion. In two hundred or three hundred years the whole world will accept it. People will forget Jupiter, and there will be no God except Christ, and no temples but Christian. Who would not wish his own happiness?"
Very beautiful! There are many other such sublime bits of prose in the this book. Do read it! It's available in the Central Library.