There seems to be three principal themes in the Diary of St. Faustina. The first is Trust. Jesus greatly desires that every soul be absolutely convinced that no matter what sins they have committed, they should never fear to approach Him to ask His forgiveness and mercy. He has made a solemn promise to all humanity, that if we turn with trust to His Mercy, He will not only forgive us, but He will also heal our wounds and grant us more graces than we can carry.
The second subject the second coming of Christ. Through His messages to St. Faustina, Jesus is preparing the world for His return and Judgement Day. He refers to it as the “awful day…of His Justice”. It is actually clear that Jesus does not even want this day of Justice. He describes it in terms which would lead you to believe that even He does not want the Day of Divine Justice. But on several occasions, at various points in the Diary, Jesus tells St. Faustina, that the second coming of Christ is near, hence why He has established the Feast of Mercy and Devotion to Divine Mercy, so that as many souls as possible can benefit from this devotion, before this dreaded day. He gives absolutely no indication as to how many years it will be before the second coming of Christ, only to say that it is near. It is on account of this revelation that He has established the new devotion to His Divine Mercy.
In our modern world, many TV documentaries have investigated religious sects who have been claiming to know that the end of the world is set for a specific date, etc. As always, these TV documentaries establish that those involved have many very serious personal problems. Discussing the end of time and the Second Coming of Christ is a sensitive matter, and we would strongly urge caution and discretion in your consideration of this issue. In truth, nobody knows and nobody can know, except God, when the second coming of Christ will take place. In fact, it is not worth worrying about because it is of no significance unless we actually knew the date on which it would occur. But one of the main purposes of the new devotion to Divine Mercy was to inform the world that this event was near and we should take advantage of the mercy of God while there was still time.
Furthermore, Jesus has given St. Faustina a direct message which would indicate that the end of time and the second coming of Christ was about to be. He told her that, Write this: before I come as the just Judge, I am coming first as the King of Mercy. Before the day of justice arrives, there will be given to people a sign in the Heavens of this sort: All light in the heavens will be extinguished, and there will be great darkness over the whole earth. Then the sign of the Cross will be seen in the sky, and from the openings where the hands and the feet of the Saviour were nailed will come forth great lights which will light up the earth for a period of time. This will take place shortly before the last day. (Diary 83) This is one of the direct messages from Jesus to St. Faustina. But even within this message there is the message of mercy, for He has now told the world that this is the sign of the end times, so when it happens, the Church will know the truth and people will still have an opportunity to seek the mercy of God. So it is a very wise, intelligent and prudent response to remain calm and equable regarding this issue.
The third strongest theme is the Diary of St. Faustina is the establishment of a new devotion to Jesus’ Divine Mercy. It is the main purpose of the revelations to St. Faustina that the Feast of Mercy be established and that the prayers and Image of Divine Mercy be promulgated throughout the world so that every soul can be forgiven and returned to a state of grace and union with God. This purpose of this devotion is to make known to the world that God is an infinitely merciful God and that no sinner, no matter how great or repulsive their sins are, will be refused forgiveness in the sacrament of confession. All Jesus asks is that we trust in His unfathomable Mercy.