Lo, I am sending my messenger
to prepare the way before me;
And suddenly there will come to the temple
the LORD whom you seek,
And the messenger of the covenant whom you desire. Yes, he is coming, says the LORD of hosts.
But who will endure the day of his coming?
And who can stand when he appears? For he is like the refiner’s fire, or like the fuller’s lye. He will sit refining and purifying silver, and he will purify the sons of Levi, Refining them like gold or like silver that they may offer due sacrifice to the LORD.
Mal 3:1-4, 23-24
Following in the steps of the prophets and John the Baptist, Jesus announced the judgment of the Last Day in his preaching. Then will the conduct of each one and the secrets of hearts be brought to light. Then will the culpable unbelief that counted the offer of God's grace as nothing be condemned. Our attitude to our neighbor will disclose acceptance or refusal of grace and divine love. On the Last Day Jesus will say: "Truly I say to you, as you did it to one of the least of these my brethren, you did it to me."
-- CCC 678