Why would Christ emphasize in our time a doctrine, the Divine Mercy, which has been part of the patrimony of the Faith from the beginning, as well as request new devotional and liturgical expressions of it?
In His revelations to St. Faustina Jesus answers this question, connecting it to another doctrine, also sometimes little emphasized, that of His Second Coming. In the Gospel the Lord shows us that His first coming was in humility, as a Servant, to free the world from sin. Yet, He promises to return in glory to judge the world on love, as He makes clear in his discourses on the Kingdom in Matthew chapters 13 and 25.
In between these Comings we have the end times or era of the Church, in which the Church ministers reconciliation to the world until the great and terrible Day of the Lord, the Day of Justice. Every Catholic should be familiar with the teaching of the Church on this matter, contained in paragraphs 668 to 679 of the Catechism of the Catholic Church. Only in the context of public revelation as taught by the Magisterium can we situate the words of private revelation given to Sr. Faustina.
You will prepare the world for My final coming. (Diary 429)
Speak to the world about My mercy ... It is a sign for the end times. After it will come the Day of Justice. While there is still time, let them have recourse to the fountain of My mercy. (Diary 848)
Tell souls about this great mercy of Mine, because the awful day, the day of My justice, is near. (Diary 965).
I am prolonging the time of mercy for the sake of sinners. But woe to them if they do not recognize this time of My visitation. (Diary 1160)
Before the Day of Justice, I am sending the Day of Mercy. (Diary 1588)
He who refuses to pass through the door of My mercy must pass through the door of My justice. (Diary 1146).
In addition to these words of Our Lord, Sr. Faustina gives us the Words of the Mother of Mercy, the Blessed Virgin,
You have to speak to the world about His great mercy and prepare the world for the Second Coming of Him who will come, not as a merciful Savior, but as a just Judge. Oh how terrible is that day! Determined is the day of justice, the day of divine wrath. The angels tremble before it. Speak to souls about this great mercy while it is still the time for granting mercy. (Diary 635).
Above from: EWTN - The Divine Mercy and the Second Coming