Were the Holy Innocents of Bethlehem True Martyrs? (Thomas Aquinas on the Baptism of Blood)

Then Herod perceiving that he was deluded by the wise men, was exceeding angry: and sending killed all the menchildren that were in Bethlehem, and in all the borders thereof, from two years old and under, according to the time which he had diligently inquired of the wise men. (Matthew 2:16, D-R)

Some have said that in the case of the Innocents the use of their free will was miraculously accelerated, so that they suffered martyrdom even voluntarily. Since, however, Scripture contains no proof of this, it is better to say that these babes in being slain obtained by God’s grace the glory of martyrdom which others acquire by their own will. For the shedding of one’s blood for Christ’s sake takes the place of Baptism. Wherefore just as in the case of baptized children the merit of Christ is conducive to the acquisition of glory through the baptismal grace, so in those who were slain for Christ’s sake the merit of Christ’s martyrdom is conducive to the acquisition of the martyr’s palm. 

Hence Augustine says in a sermon on the Epiphany (De Diversis lxvi), as though he were addressing them: “A man that does not believe that children are benefited by the baptism of Christ will doubt of your being crowned in suffering for Christ. You were not old enough to believe in Christ’s future sufferings, but you had a body wherein you could endure suffering of Christ Who was to suffer.”  

– Summa theologiae II-II q. 124, a. 1, ad. 1.