A secret gathering of Christian worshipers assembles, ready to be baptised into the new faith.
Chorus of Christians:
O voûte obscure, ô voûte obscure / "O dark vault, o immense vault, tombs where peace reigns".

As they are about to go into the catacombs, Polyeucte, the principal magistrate of Mytilene, is approached by his friend Néarque, the Christian leader, who questions him on his faith: "New Christian, has the God whose law we follow put faith and bravery in your heart?",
he asks, continuing to pursue Polyeucte's feelings about how he would reconcile his wife's beliefs with his own.
Polyeucte assures him that when his wife was deathly ill, his local gods were deaf to his pleas, but the Christian God provided a signal by means of a clap of thunder and His voice saying "I am waiting for you".
Aria: Que l'onde salutaire, s'épanche sur mon front!
Et les maux de la terre, pour moi disparaîtront! / "
Let the water of salvation / Flow over my brow / And the evils of the earth / For me will disappear!"
Néarque pleads for heaven and the angels to receive his friend and the two men enter the catacombs.
As they leave, a Christian rushes in to inform them that the governor's soldiers are on their way.
Unafraid, Polyeucte declares "Let's go!
God expects us!"

Pauline arrives to pray at her mother's tomb.
The accompanying soldiers depart, and she offers a sacrifice.
Along with the group of young girls who have accompanied her, they sing a hymn to Proserpine, after which she orders all to leave her.
Alone, Pauline sings to her dead mother.
Aria Toi, qui lis dans mon coeur, ô ma mere! /
O toi, qui fus témoin de l'amour de Sévère, de ces nœuds par toi-même approuvés! / "
You, who see into my heart, oh mother!
Oh you who witnessed the love of Sévère".
In her prayer, she tries to balance her feelings for her dead lover with those for her new husband.
However, as she prays, she hears the sound of Christian singing coming from the catacombs.
 Chorus: O toi, notre père, qui règnes sur terre, comme dans les cieux / "O you, our father, who reigns on earth as in heaven".
She is horrified at hearing the words of "that ungodly sect".

As the Christians leave the catacombs, Polyeucte is surprised to find Pauline there; she is equally shocked and, while he proclaims his faith, she maintains her skepticism.
Néarque and the group kneel in prayer for her, while she pleads to Polyeucte to abandon his beliefs.
He continues to refuse as the pleading goes on; finally Pauline threatens to reveal the sect's existence to her father, but she is rebuffed.
Together the couple plead their cases: his to call upon God's mercy while hers is to beg him to keep silent.

Act finale: Duet and ensemble, first Pauline: Si tu m'aimes, silence! / "
Silence, if you love me!";
then Polyeucte: Objet de ma constance, amour de ton époux/ "You are the object of my constant love as your husband"; Néarque and the Christians: "God is our defense; God will watch over us".
A secret gathering of Christian worshipers assembles, ready to be baptised into the new faith. Chorus of Christians:
O voûte obscure, ô voûte obscure / "O dark vault, o immense vault, tombs where peace reigns".
As they are about to go into the catacombs, Polyeucte, the principal magistrate of Mytilene, is approached by his friend Néarque, the Christian leader, who questions him on his faith: "New Christian, has the God whose law we follow put faith and bravery in your heart?", he asks, continuing to pursue Polyeucte's feelings about how he would reconcile his wife's beliefs with his own. Polyeucte assures him that when his wife was deathly ill, his local gods were deaf to his pleas, but the Christian God provided a signal by means of a clap of thunder and His voice saying "I am waiting for you". Aria: Que l'onde salutaire, s'épanche sur mon front! Et les maux de la terre, pour moi disparaîtront! / "Let the water of salvation / Flow over my brow / And the evils of the earth / For me will disappear!" Néarque pleads for heaven and the angels to receive his friend and the two men enter the catacombs. As they leave, a Christian rushes in to inform them that the governor's soldiers are on their way. Unafraid, Polyeucte declares "Let's go! God expects us!"
Pauline arrives to pray at her mother's tomb. The accompanying soldiers depart, and she offers a sacrifice. Along with the group of young girls who have accompanied her, they sing a hymn to Proserpine, after which she orders all to leave her. Alone, Pauline sings to her dead mother. Aria Toi, qui lis dans mon coeur, ô ma mere! / O toi, qui fus témoin de l'amour de Sévère, de ces nœuds par toi-même approuvés! / "You, who see into my heart, oh mother! Oh you who witnessed the love of Sévère". In her prayer, she tries to balance her feelings for her dead lover with those for her new husband. However, as she prays, she hears the sound of Christian singing coming from the catacombs.  Chorus: O toi, notre père, qui règnes sur terre, comme dans les cieux / "O you, our father, who reigns on earth as in heaven". She is horrified at hearing the words of "that ungodly sect".
As the Christians leave the catacombs, Polyeucte is surprised to find Pauline there; she is equally shocked and, while he proclaims his faith, she maintains her skepticism. Néarque and the group kneel in prayer for her, while she pleads to Polyeucte to abandon his beliefs. He continues to refuse as the pleading goes on; finally Pauline threatens to reveal the sect's existence to her father, but she is rebuffed. Together the couple plead their cases: his to call upon God's mercy while hers is to beg him to keep silent.
Act finale: Duet and ensemble, first Pauline: Si tu m'aimes, silence! / "Silence, if you love me!"; then Polyeucte: Objet de ma constance, amour de ton époux/ "You are the object of my constant love as your husband"; Néarque and the Christians: "God is our defense; God will watch over us".
Les martyrs