Alma 42:13

therefore, according to justice, the plan of redemption could not be brought about, only, on conditions of repentance of men in this probationary state; yea, this preparatory state; for except it were for these conditions, mercy could not take effect except it should destroy the work of justice. Now the work of justice could not be destroyed: if so, God would cease to be God.

According to what’s fair, God’s plan to rescue people could only work if people repented in this life. Without this, God’s mercy couldn’t work. It would destroy what’s fair. What’s fair can’t be destroyed. If it was, God would stop being God.

According to justice, the plan of redemption could only work if people repented during this testing period, this time of preparation. Without that condition, mercy couldn’t operate—it would undermine justice. And justice cannot be undone. If it could be, God would cease to be God.