24 Let every one then abhor sin, which defiles the mystical members of our Redeemer; but if anyone unhappily falls and his obstinacy has not made him unworthy of communion with the faithful, let him be received with great love, and let eager charity see in him a weak member of Jesus Christ. For, as the Bishop of Hippo remarks, it is better "to be cured within the Church's community than to be cut off from its body as incurable members." "As long as a member still forms part of the body there is no reason to despair of its cure; once it has been cut off, it can be neither cured nor healed."
Source: Mystici Corporis Christi (Vatican.va)