The Saintess Was Switched at Birth

Just before making her high school debut, Nagi wakes up to find herself deep inside a forest. It seems she has somehow switched places with Rio, a girl from another world whom Nagi has seen repeatedly in her dreams since childhood! Normally, those dreams would end quickly—but this time, she can’t wake up no matter what she does!? Lost and helpless, Nagi is discovered by an elite Mage Knight Order. When they learn that she is a Saint, they ask for her help in subjugating magical beasts. However, Nagi can’t sing the “holy hymns” that allow a saint to use her full power. So what’s her greatest weapon instead…? Her fists, of course!? Punching her way through endless troubles and purifying everything in her path, Nagi goes on to make a name for herself as the strongest “living weapon” in history— thus begins a high-energy cure fantasy starring an out-of-control Saint!

https://www.cmoa.jp/title/321041/