Today is the Solemnity of the Sacred Heart of Jesus. It is an old and extremely popular devotion; although it has faded somewhat during the 20th century, it appears to me to be reviving. Some have said that the Divine Mercy Devotion is a ’reboot’ of the Sacred Heart Devotion; a close comparison reveals much similarities and
It is concerned with the intense and overwhelming love that Jesus Christ has for humanity, so much so that He gave Himself up for us as a propitiatory sacrifice for our sins and remains with us today in the Real Presence of the Blessed Sacrament. A component of the devotion is our love for Him in return, a love that has reparative aspects as we love and offer sacrifices to Him on behalf of this that do not love Him (or know of His love.) More-so, the reparative elements are in light of blasphemies, sacrileges and other sins of hate against Our Lord. We love Him in return for His love for us, and our love makes up for the lack of love others have for Him, often manifesting in acts of hate.
The devotion began in the 17th Century when Our Lord appeared to St. Margaret Mary Alacoque, an Augustinian nun renowned for her piety and incredible humility. He grieved over the growing coldness of human hearts toward Him and in His love and Divine Mercy. Much of this coldness arose as a result of the heresy of Jansenism, which essentially teaches that God is harsh and judgmental, and few are saved since humanity is so depraved and evil due to sin. For some reason Jansenism became very popular in Europe, particularly in France. I think Calvinism was happier.
I looked up Jansenism in the Catholic Encyclopedia; apparently Jansenism arose from a text written by Archbishop Cornelius Jansen of Ypres, Belgium, that wasn’t published until some time after his death. He, himself, appears to have died in the graces of the Church and allegedly wrote a testament submitted his work to the judgment of the Church. His executor unsuccessfully tried to suppress the work, he failed and copies of it were published and distributed widely, attracting devotees. You can read all of that here and decide for yourself: Jansenius and Jansenism"The Stations of the Cross for Alcoholics" and "The Recovery Rosary: Reflections for Alcoholics and Addicts" (Thank you!!)