Does Matthew 18:18 teach that Jesus promised to back up the decrees of the disciples and perhaps those of their successors? Not according to Craig Keener in the Basics of Biblical Greek Grammar (William D. Mounce). "The matter is not quite so simple; the actions described in heaven are future perfect passives-which could be translated 'will have already been bound in heaven...will have already been loosed in heaven.' In other words, the heavenly decree confirming the earthly one is based on a prior verdict."
The language being used here is similar to a Jewish law court where elders in the synogogues made decisons on legal issues. "Many Jewish people believed that the authority of Heaven stood behind the earthly judges when they decided cases based on a correct understanding of God's law. (This process came to be called 'binding and loosing')." By obeying God's law, the earthly court upheld what was decreed by the heavenly court (121).
This is the will of God being carried out through the church based off of what has already been done in heaven. It is a matter of the church recognizing the impenitent heart of an individual who has already been seperated from God and reflecting this in the fellowship of the church. When a person refuses to turn from their sin (even after loving confrontation), the church has the responsibility to make the reality of the sin before God clear to everyone.
What are the implications for the church's authority? Church authority must be looked at in terms of recognizing what God has already decreed and in the context of this passage, this must be done with great care! The passage does not assure us that our judgments will be infallible, but if we follow the process laid out (concerning witnesses and going to the person in private) in the will of God (see John 16 also), then the heavenly court will be reflected on earth perhaps even with supernatural action (Mtt 18:19-20 and 1 Cor 5:4-5,13) all of this was of course assuming a correct interpretation of the Scriptures (Colin Brown, Dictionary of NT Theo vol.3 p.781).
This understanding poses a challenge to those Protestant churches that deny the church has any authority. The passage is also often used by the RC and EO as part of a case for their own perspectives on church authority as well. The passage however may not be used in support of church infallibility.