Explanation of a Saint

Saints are an existence called "Child of God" and are powerful individuals on the Magic Side. Less than 20 of them exist in the world. Saints possess an innate bodily characteristic, allowing them to assume a portion of the Son of God's attributes, blessing them with superhuman strength, speed, sense, healing and the capacity to channel larger amounts of mana compared to normal magicians. The structure behind Sainthood is based in Idol Theory, where an imitation of a more powerful being or item can retain a small portion of the original's strength. Humans are said to be made in the image of God and a Saint is said to be a higher replica that can draw out more power. As Saints are still human, there is a limit on the amount of power that is usable through Idol Theory. Moreover, if a Saint released 100% of their strength, the high pressure would exceed the limits of the human body and cause the Saint to self-destruct. Due to their nature, Saints are vulnerable to attacks that use symbolism related to the Son of God's crucifixion.

“There’s this hidden Christian organization in Japan called the Amakusa Christian Church, see. Before Kanzaki nee-chin was born, it was decided she would rise to the top of their group and be their priestess. And that she was one of God’s chosen ones, a Saint blessed with the Stigma, of whom there are less than twenty in the world.”
(Toaru Majutsu no Index Volume 4, pg 100)

Tsuchimikado turned his gaze to him, amazed, from out of his blue sunglasses. “Listen here. Kanzaki nee-chin has the Stigma, got it? She’s a human weapon—no, a weaponized Saint. Can you really classify her as ‘just a girl’?” “...What did you say? Weaponized...Saint?” “It’s just how it sounds. Kami-yan, I explained about idol-worshipping religions yesterday, right? The stuff about even if the cross on a church roof is a replica, it’s still got a degree of power as long as its shape and role are the same.” Tsuchimikado spoke quickly, in consideration for Kanzaki and Misha, who had gone on ahead. “That applies to ‘replicas of God,’ too. Humans were designed in the form of God, so a human ‘replica’ having the power of the original God is possible. Of course, that only applies to a select few people who are like God. Such as her, who was proclaimed at birth to be a Saint within whom would reside godlike power. While her Stigma, her proof of Sainthood, is unleashed, she can temporarily use superhuman power. Don’t you think she could take out an entire castle by herself right now?”
(Toaru Majutsu no Index Volume 4, pg 105-106)

“Though there hasn’t been any specific information, he obviously has the strength worthy of a member of God’s Right Seat and what seems to be the power of a Saint, so it’d be great if we can get the Priestess’s assistance.” The Priestess that she was talking about was Kanzaki Kaori. She was one of the less than twenty Saints in the world, and had managed to survive a fight against a real archangel. That was truly one achievement she could brag about.
(Toaru Majutsu no Index Volume 16, pg 15)

Normally, the power given to a Saint couldn’t be used entirely. Since they were people who had similar body characteristics with the Son of God, Saints were those who were said to have inherited his power; even if it was just a part of it, in the end, it was not something that a human could grasp. They had a part of that power, but trying to grasp a part of that part of power was already tough for a Saint. This was the real identity of a Saint. No matter how they built this up, there would be a divide that they couldn’t surpass. If one had to describe it, it would be like finally getting hold of a power only for it to scatter like fog. Thus, there was a limit to using the power created through Idol Theory with their own willpower. However, this wasn’t completely bad. If Saints could use such power at 100%, the high pressure would cause the Saint’s body to self-destruct. This was more of a self-defense instinct than magic, since when they were babies, they controlled that power without knowing anything about magic.
(Toaru Majutsu no Index Volume 16, pg 158-159)

All of them were a great force that represented Britain and they were all either a Saint or the possessor of an equivalent level of power. They would occasionally move at greater than the speed of sound, they would occasionally singlehandedly wipe out an army, and their power was said to be proportional to a nuclear weapon.
(Shinyaku Toaru Majutsu no Index Volume 10, pg ?)