Great question, I understand why it can be confusing. To clarify, here is the breakdown of the terms used in the Japanese source:
For the MC (Satoru): In the heat of battle, Satoru shouts 'Seija ni demo nandemo natte yaru yo!' (聖者にでも なんでも なってやるよ!). The term 'Seija' (聖者) specifically translates to 'Saint' or 'Holy Person'. He says this as a bold declaration that he’s willing to become whatever it takes to protect those he cares about.
For Fortuna: While she is often associated with the role of a Saint, she actually refers to Satoru as 'Kyuseishu' (救世主). This term specifically means 'Savior' or 'Messiah' (the one who saves the world).
So why the shift?
Satoru is calling himself a Saint (聖者) in a 'fine, I'll do it' kind of way to mock the gravity of the situation. Meanwhile, Mictran acknowledges his resolve by saying he looks like a True Savior (救世主) because he fights for his 'precious ones' rather than just some abstract duty.