I was amazed to read recently where a Catholic priest theologian said that the Church is not infallible and that it has changed its doctrines. Apparently he felt Jesus' words in Matthew 18:18-19 ("Whatever you bind on earth shall be bound in heaven, and whatever you loose on earth shall be loosed in heaven. Again, I say to you, if two of you agree on earth about anything for which they are to pray, it shall be granted them by my heavenly Father") mean that the Church can change its doctrines as times change.
No, the Church cannot change its doctrines no matter how badly some theologians may want it to or how loudly they claim it can. The doctrines of the Catholic Church are the deposit of faith revealed by Jesus Christ, taught by the apostles, and handed down in their entirety by the apostles to their successors. Since revealed truth cannot change, and since the deposit of faith is comprised of revealed truth, expressed in Scripture and Sacred Tradition, the deposit of faith cannot change.
While it's certainly true that our Lord's words to the apostles in Matthew 18:18-19 grant authority to the apostles to "bind" the members of the Church to believe the doctrines of the Church ("He who listens to you listens to me. He who rejects you rejects me and the one who sent me" [Luke 10:16]), the "loosing" spoken of in Mathew. 18:18 does not mean the apostles can modify doctrine.
The Church does not have the power to do the impossible, to change or delete divinely revealed truth which forms the deposit of faith. Rather, the concept of loosing, as it pertains to the apostles and their successors, has more to do the Church's ability to dispense individuals or the whole Church from observing certain ecclesiastical disciplines. There are many examples of this authority to bind and loose in the arena of Church discipline. Here are a few:
- In the early Church married men were permitted to be ordained as priests in the West. This custom was changed in the tenth century and since then, in the Latin Rite, candidates for the priesthood must be celibate.
- Until recent years it was forbidden under pain of mortal sin to eat meat on Fridays. The Church has "loosed" this discipline and now allows the faithful to eat meat on Fridays provided some other act of mortification is performed.
- Prior to Vatican II, the pre-Communion fast extended from midnight until the time one received Communion; no food or water were to be consumed. This discipline was relaxed first to a three-hour fast and then to the one-hour fast the Church now observes.
- Matthew 18 also refers to the Church's authority to bind and loose with regard to sin. Every priest has the authority of Christ to "loose" (absolve) penitents from their sins through the sacrament of confession (Jn 20:21-22, 2 Cor 5:18-20). The priest also has the obligation, in rare cases when he sees no evidence of contrition or an unwillingness on the part of the penitent to stop committing sin, to "bind" someone in their sins by refusing to grant him absolution until he evinces genuine contrition.
- The faithful may gain indulgences through corporal and spiritual acts of charity, certain prayers, and pilgrimages, and are thus, by the authority of the Church's power to bind and loose, able to receive partial or complete remission of all temporal punishment due to sin. Through indulgences the Church may loose Christians from the duty of penance which would otherwise need to be performed.
- Certain forms of excommunication may be "loosed" only by a bishop or, in graver circumstances, only by the Holy See.
- Priests and religious who request it can be "loosed" (dispensed) from their vows of celibacy (and, in the case of religious, the vows of poverty and obedience).
None of these issues deals with doctrine as such (since doctrine is unchangeable) but with Church discipline, government, and penance.