Mateo 18:16

But if he hear thee not, take with thee one or two more, that at the mouth of two witnesses or three every word may be established.

Yes, these words were spoken before Pentecost, but they are nevertheless binding upon the church and Christians of all ages. They were uttered specifically for the purpose of outlining procedures for discipline in "the church," as attested by the use of such terminology by the Saviour. See the following verse.