Explanation of "Double-Saint" Equilibrium

This spell wouldn’t just increase movement, it also prevented the muscles from exceeding what humans could endure and get destroyed, making sure that while the speed drastically increased, the entire body balance wouldn’t be lost. This was a crystallization of all sorts of spells. If one wanted to go even further than that, or add in something else, the balance between all these spells would break down. Using the current pieces and piecing them together like a jigsaw puzzle, it was impossible to add a new piece in. This was the limit of a Saint who mainly used close-ranged combat. Maybe Acqua had some body defense spell that was even more refined than this. Though Kanzaki thought of a few possibilities herself, no matter what, she failed. As expected, there would be a problem if she powered up to his level. If she used a similar power to that of Acqua’s. If anyone tried to use the power that Acqua used, at that moment, an ordinary person’s body would have eroded, whether physically or magically. (Acqua’s...power is...) 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. However. (...Acqua, he doesn’t have the limit of being...a Saint...?...That power...has it already overcome what...a human can already control...?) Needless to say, besides being a Saint, Acqua had the power of God’s Right Seat. The reason for him to be called Acqua of the Back was because he had the element of Archangel Gabriel. Normally, one would think that it was just a natural addition of power, but in reality, this increased power doubled the burden on him. Yes. What was amazing was that Acqua had already controlled 200% of that power and still hadn’t gone amok, his facial expression not changing at all. (...That’s impossible. This isn’t something that can be excused through a word like quality or genius. These two incompatible elements of a Saint and God’s Right Seat are both in the same body. This is already impossible on its own...) The thing called talent was a forceful yet acceptable one. But this was different. Since Kanzaki was within this level, she understood. Talent and ability, those two things weren’t such convenient things in reality. (...Is there anything...) Kanzaki heard a small sound.
(Toaru Majutsu no Index Volume 16, pg 157-159)

The reason why Acqua would defend against the Saint Destroyer with all he had was because after taking the hit, he wouldn’t just lose some part of his power or be unable to use his power for several seconds. “The Saint Destroyer forcefully destroys anyone with a body similar to that of the Son of God, robbing the internal power and causing the Saint to be unable to move. Normally, you would return to normal after several seconds, but for you Acqua, who has both the attributes of a Saint and the Virgin Mary, what the outcome will be obvious. You will self- destruct!!” The spell he had cast used the delicate naturally-born-with attribute of a Saint being superior to a human. In other words, this wasn’t something that could be done through human means. Normally, a godlike technique would cause everything to destruct if the equilibrium was destroyed. Like an engine that could run up to 1000 kph, as there was such a powerful force, it needed delicate care. Thus, Acqua had used his full power to defend against it.
(Toaru Majutsu no Index Volume 16, pg 171-172)

“The symbols of the crucifixion of the Son of God!”Since he had inherited a part of a Saint’s power, he should have inherited the weaknesses as well. Even so, if this was enough to beat a Saint, nobody would need to work so hard. Honestly, to an ordinary Saint like Kanzaki, it wouldn’t have much effect on her. However. Acqua of the Back was a special Saint. Compared to the less than twenty Saints in the entire world, he had an even rarer attribute. He had both the power of a Saint and the Virgin Mary at the same time. At the same time when he had this tremendous power, he had to balance these two powers. This was why Kanzaki Kaori would choose the symbol of execution. The execution spell may seem unrelated to the Virgin Mary, but in this situation, that was not so. Yes. For the Virgin Mary was treated as the greatest saint in Christian history. “—!!” At this moment, Kanzaki sensed something that exceeded physical concepts floating up around Acqua. This was a change that even a normal Saint could sense. In other words, Acqua was... “He’s wavering.” Kanzaki spoke with complete belief. With Acqua at center, what existed were the powers of a Saint and the Virgin Mary. Both of them were affected by the outside environment, and there was an intense fight inside his body, even letting out an ugly cry. If it was now, they could do it. Thus, Kanzaki Kaori shouted from deep within her heart. “Everything is in place! Those wielding the Lance of Longinus, now’s the time for the execution!!”
(Toaru Majutsu no Index Volume 16, pg 172-173)