The Resurrection of Jesus was the most important event in the history of the world. When the Lord appeared in the 'Upper Room' to the apostles pointing out His wounds, it was to demonstrate He was the Crucified Christ, and to show He had resurrected from the dead. The final proof that He was God. He then bestowed on the apostles the power to forgive sins, which was in effect Divine Mercy coming into the world.
It was the beginning of the Church on earth. In 1931 He appeared as the same image in another room, this time the room of a young nun, again showing His wounds, emphasising the power of those wounds as rays from a font of reconciliation and renewal, rays of love and mercy. This was Divine Mercy, His victory on the Cross, His resurrection, coming into to a very sinful world once again. A world that was about to plunge man into the depths of depravity in a horrific world war.
He asked her to teach the world again of why he died on the Cross to give sinners another chance of obtaining God's Divine Mercy, and pardon their sins if they returned to living by God’s commandments. He was also reminding man that His death on the Cross, Divine Mercy was for all mankind (And there are other sheep I have that are not of this fold, these I have to lead as well. They too will hear My voice, and then there will be one flock and one Shepherd. John 10:11 – 18)
The commandments of God are built into every human born, in the form of a conscience that was given them at birth, it is the spirit of God within them, if you deny your conscience, you deny God.
The image that appeared to the apostles was the resurrected Christ. The Resurrection was the start of the Christian Church. The image that appeared to Saint Faustina was the same image that started the Christian Church. His appearance to Saint Faustina as the same image was to renew the Church, to renew belief in the Cross, in the Resurrection, in mans salvation.
In the beginning he said “I have not come to call the righteous, but to call sinners to repentance”. (Luke 5:32) For the same reason He appeared again on earth to Saint Faustina, to save sinners from eternal damnation. His message of Divine Mercy is a warning to mankind to exercise their conscience, come back to God and live by the Commandments.
Who are the sinners? – sinners are those who offend human nature, God’s creation, and the dignity of man.