Where's the theological proof for it?
Pope Benedict XIV
Doctrina de servorum Dei beatificatione et beatorum canonizatione lib. 3 cap. 15 "De requisitis in martyre necessariis ante martyrium":
Martyrium dicitur baptismus sanguinis Marc. 10. In adultis est actus fortitudinis, et caritatis : quoad infantes, sufficit occisos fuisse ex odio in Christum, ut martyrium suppleat vices baptismi aquæ, deleatque ex opere operato peccatum originale quoad culpam, et poenam, ut docet S. Thomas (2, 2, qu. 124, art. 1 ad 1)
What constitutes killing out of hatred of Christ (
ex odio in Christum) is not as restrictive as some may think. As Billot, S.J. (
De ecclesiæ sacramentis t. 1 p. 242) mentions, a "formal hatred of Christ and His religion" is not necessary, as some emperors who martyred did so only out of motives of ignorance of Christianity, upholding civil law, or public opinion; however, they did target Christians specifically.
Cdl. Cajetan, O.P., in
his commentary on
III q. 68 a. 11 ("Whether a child can be baptized while yet in its mother's womb?") is even more explicit
, saying that even
in utero infants can be martyrs like the Holy Innocents:
Si enim propter Christum infans in utero occideretur, martyr esset, non minus quam Innocentes.