Whatever you bind on earth shall having been bound in heaven... and vice versa.

A. T. Robertson, one of this century's leading Greek scholars, also comments on Matthew 16:19: "To `bind' in rabbinical language is to forbid, to `loose' is to permit. Peter would be like a rabbi who passes on many points. Rabbis of the school of Hillel `loosed' many things that the school of Schammai `bound.' The teaching of Jesus is the standard for Peter and for all preachers of Christ. Note the future perfect indicative..., a state of completion. All this assumes, of course, that Peter's use of the keys will be in accord with the teaching and mind of Christ."2 Dr. Robertson's comment about the use of the future perfect tense is important. If we were to translate the passage very literally (though awkwardly in English), it would read "...whatever you loose on earth shall having been loosed in heaven." This shows that the disciples were not unilaterally to decide a matter, thus binding "heaven" to their decision. It means that their decision, as Dr. Robertson suggests, will be in line with what already was God's mind on the issue.

-http://cicministry.org/commentary/issue1.htm