10 If any man receive not Baptism, he has not salvation; except only Martyrs, who even without the water receive the kingdom. For when the Saviour, in redeeming the world by His Cross, was pierced in the side, He shed forth blood and water; that men, living in times of peace, might be baptized in water, and, in times of persecution, in their own blood. For martyrdom also the Saviour is wont to call a baptism, saying, Can you drink the cup which I drink, and be baptized with the baptism that I am baptized with? And the Martyrs confess, by being made a spectacle unto the world, and to Angels, and to men; and you will soon confess:— but it is not yet the time for you to hear of this.
Source: Catechetical Lectures (New Advent)