It's not "mercy" because from Adam until when Jesus declares "let him...be unjust still" and "let him...be holy still", the only hope for the salvation of the sinner is that he accept by faith that he is saved by "grace through faith"..."the just shall live by faith".
You say "by faith for some, by sight for others"...
...when last day sinners see all the Christians and babies vanish in the "secret rapture" or some other supernatural event that renders faith no longer necessary because for the first time, sinners will not only have their evidence God exists...but also a Second Chance to get right with God that was denied everyone else up until that time, right? I guess all that "watch and be ready" and "Today, if you hear his voice..." stuff doesn't matter, right? Sorry, "God is no respecter of persons". No Second Chance, only a Second Coming. Not "mercy", injustice.
God grants mercy to whoever he pleases. And he also denies mercy to whoever he pleases. Mercy has nothing to do with justice, we all deserve to die according to the Torah of God. Mercy means that God does not render the death we deserve, which will be the case with some at the second coming of the Messiah, which will be the lame, as it is written, "I will gather the lame, and make a strong nation out of them, and rule over them from Mount Zion". Even so, did the Israelites not witness the power of God with plagues against Egypt and deliverance from slavery, yet because of their disobedience and even lack of belief, as was seen with Moses, were prevented from entering the land of their inheritance? Belief is something that will never disappear, neither in this aeon, nor in the next aeon, nor in the aeons of the new heaven and new earth. Belief, hope, love always remain.