"Anyone who desires to receive Christ in Eucharistic communion must be in the state of grace. Anyone aware of having sinned mortally must not receive communion without having received absolution in the sacrament of penance." Source: Catechism of the Catholic Church #1415
"Holy Communion is incorporation not only to the life of Christ, but also to His death. This second aspect is sometimes forgotten. St. Paul mentions it: 'So it is the Lord's death that you are heralding, whenever you eat this bread and drink this cup, until he comes' (I Corinth. 11:26)... What He did in [...]
"In the Church, gathering particularly today in a special way around the Eucharist and desiring that the authentic Eucharistic community should become a sign of the gradually maturing unity of all Christians, there must a lively-felt need for penance, both in its sacramental aspect, and in what concerns penance as a virtue." Source: Pope [...]
"The Eucharist and Penance thus become in a sense two closely connected dimensions of authentic life in accordance with the spirit of the Gospel, of truly Christian life. The Christ who calls to the Eucharistic banquet is always the same Christ who exhorts us to penance and repeats his 'Repent'." Source: Pope St. John [...]
"We have said that it is absolutely necessary to give one's mind to God, that the most dangerous struggles are those of the mind, that tepidity of life has its source in sloth and indecision in choosing God and rejecting evil. We have said that, at once and unhesitatingly, thoughts that have even the appearance [...]
"If you take not my word for this marvelous virtue (penance/mortification), try it yourselves, for a time, at least; and when you shall have had a taste of it, ah! you will not give it up. You must become convinced of it, enthusiastic over it, for to do anything well, to arrive at the possession [...]
"There are two ways of honoring Our Lord by penance; the one is inspired by negative love, the other by positive love. By the first, we prevent evil or we correct it. It is necessary, but it makes us act only through the strict duty of Christian justice. For the practice of this kind of [...]
"True, it is hard, and there are few who persevere in it and become saints. They mortify themselves for a time while the passions are still to be feared; and when peace reigns they cease to do so. From this it comes that there are so many of merely ordinary virtue, and that they never [...]
"The first purification to be made is from sin; the means whereby to make it, the Sacrament of Penance. Seek the best Confessor within your reach, use one of the many little books written in order to help the examination of conscience." Source: St. Francis de Sales, Introduction to the Devout Life, Random House, [...]
"That many do not advance in the Christian progress because they stick in penances, and particular exercises, while they neglect the love of God, which is the end. That this appeared plainly by their works, and was the reason why we see so little solid virtue." Source: Brother Lawrence, The Practice of the Presence [...]