15 Make ready then the vessel of your soul, that you may become a son of God, and an heir of God, and joint-heir with Christ; if, indeed, you are preparing yourself that you may receive; if you are drawing near in faith that you may be made faithful; if of set purpose you are putting off the old man. For all things whatsoever you have done shall be forgiven you, whether it be fornication, or adultery, or any other such form of licentiousness. What can be a greater sin than to crucify Christ?
Yet even of this Baptism can purify. For so spoke Peter to the three thousand who came to him, to those who had crucified the Lord, when they asked him, saying, Men and brethren, what shall we do? For the wound is great. You have made us think of our fall, O Peter, by saying, You killed the Prince of Life. What salve is there for so great a wound? What cleansing for such foulness? What is the salvation for such perdition? Repent, says he, and be baptized every one of you in the name of Jesus Christ our Lord, for the remission of sins, and you shall receive the gift of the Holy Ghost. O unspeakable loving-kindness of God!
They have no hope of being saved, and yet they are thought worthy of the Holy Ghost. You see the power of Baptism! If any of you has crucified the Christ by blasphemous words; if any of you in ignorance has denied Him before men; if any by wicked works has caused the doctrine to be blasphemed; let him repent and be of good hope, for the same grace is present even now.
Source: Catechetical Lectures (New Advent)