“But when a righteous man turns away from his righteousness and commits iniquity and does the same abominable things that the wicked man does, shall he live? None of the righteous deeds which he has done shall be remembered; for the treachery of which he is guilty and the sin he has committed, he shall die.”
Short answer
The Church teaches that God does not forsake anyone he has justified, unless that person first forsakes him. A justified person remains free. He can turn away from God by mortal sin and so lose the grace he received, and Scripture gives this warning to believers. The one who falls can be justified again through repentance and the sacrament of Penance.
Drafted from primary sources. Every quotation is checked against the page it links to.
Where this teaching appears on the Offshoots timelines: 1559, The elect cannot lose salvation ("once saved, always saved").
16 quotations in date order
“Every branch of mine that bears no fruit, he takes away, and every branch that does bear fruit he prunes, that it may bear more fruit. … Abide in me, and I in you. … If a man does not abide in me, he is cast forth as a branch and withers; and the branches are gathered, thrown into the fire and burned.”
“They were broken off because of their unbelief, but you stand fast only through faith. So do not become proud, but stand in awe. For if God did not spare the natural branches, neither will he spare you. Note then the kindness and the severity of God: severity toward those who have fallen, but God's kindness to you, provided you continue in his kindness; otherwise you too will be cut off.”
“For if we sin deliberately after receiving the knowledge of the truth, there no longer remains a sacrifice for sins, but a fearful prospect of judgment, and a fury of fire which will consume the adversaries. … How much worse punishment do you think will be deserved by the man who has spurned the Son of God, and profaned the blood of the covenant by which he was sanctified, and outraged the Spirit of grace?”
“For if, after they have escaped the defilements of the world through the knowledge of our Lord and Savior Jesus Christ, they are again entangled in them and overpowered, the last state has become worse for them than the first. For it would have been better for them never to have known the way of righteousness than after knowing it to turn back from the holy commandment delivered to them.”
“But often shall you come together, seeking the things which are befitting to your souls: for the whole time of your faith will not profit you, if you be not made perfect in the last time.”
“We ought not, therefore, as that presbyter remarks, to be puffed up, nor be severe upon those of old time, but ought ourselves to fear, lest perchance, after [we have come to] the knowledge of Christ, if we do things displeasing to God, we obtain no further forgiveness of sins, but be shut out from His kingdom. And therefore it was that Paul said, For if [God] spared not the natural branches, [take heed] lest He also spare not you,”
“Whosoever that confessor is, he is not greater, or better, or dearer to God than Solomon, who, although so long as he walked in God's ways, retained that grace which he had received from the Lord, yet after he forsook the Lord's way he lost also the Lord's grace. And therefore it is written, Hold fast that which you have, lest another take your crown. … But assuredly the Lord would not threaten that the crown of righteousness might be taken away, were it not that, when righteousness departs, the crown must also depart.”
“But of those sheep of which the apostle says, The Lord knows them that are His; … there is none of such sheep as these that the wolf seizes, or the thief steals, or the robber slays. He, who knows what He gave for them, is sure of their number. And it is this that He says: No one shall pluck them out of my hand;”
“It is, indeed, to be wondered at, and greatly to be wondered at, that to some of His own children — whom He has regenerated in Christ — to whom He has given faith, hope, and love, God does not give perseverance also, when to children of another He forgives such wickedness, and, by the bestowal of His grace, makes them His own children.”
“We can, accordingly, consider charity in three ways: first on the part of the Holy Ghost, Who moves the soul to love God, and in this respect charity is incompatible with sin through the power of the Holy Ghost, Who does unfailingly whatever He wills to do. … Thirdly, charity can be considered on the part of its subject, which is changeable on account of the free-will. … the charity of the wayfarer does not so fill the potentiality of its subject, because the latter is not always actually directed to God: so that when it is not actually directed to God, something may occur whereby charity is lost.”
“For God forsakes not those who have been once justified by His grace, unless he be first forsaken by them.”
“As regards those who, by sin, have fallen from the received grace of Justification, they may be again justified, when, God exciting them, through the sacrament of Penance they shall have attained to the recovery, by the merit of Christ, of the grace lost: for this manner of Justification is of the fallen the reparation: which the holy Fathers have aptly called a second plank after the shipwreck of grace lost.”
“[If] any one saith, that a man once justified can sin no more, nor lose grace, and that therefore he that falls and sins was never truly justified; … let him be anathema.”
“He is not saved, however, who, though part of the body of the Church, does not persevere in charity. … All the Church's children should remember that their exalted status is to be attributed not to their own merits but to the special grace of Christ. If they fail moreover to respond to that grace in thought, word and deed, not only shall they not be saved but they will be the more severely judged.”
“Mortal sin is a radical possibility of human freedom, as is love itself. It results in the loss of charity and the privation of sanctifying grace, that is, of the state of grace. If it is not redeemed by repentance and God's forgiveness, it causes exclusion from Christ's kingdom and the eternal death of hell, for our freedom has the power to make choices for ever, with no turning back.”