Do you believe what the Bible says about Jesus dying "once for all"? Do you believe he is the Savior of all mankind throughout the last two millennia? If not, why not?
If you do believe that Jesus is the Savior of us today, why wouldn't his truth given to his first disciples not be pertinent to we who are his disciples now? If the teachings Jesus gave to his disciples in the Gospels, and in the rest of the NT through divine inspiration, were generally universally-applicable, if those teachings have an enduring, objective authority over all of his disciples throughout time, then we can say to each other today, "This is what God's word says, so you ought to believe and do it." The authority to do so, doesn't originate in us, though, right? It is an authority to which we point when we say to each other, "You ought to believe and do this." God's word is authoritative because it is His word, not because, when I cite it, I'm a pastor, or discipler, or teacher.
The Early Church recognized that the teachings of Christ and the teachings of those he personally, directly made his apostles, held general truth applicable to all throughout time. And so, there was much copying of the letters and Gospels of the NT that went on. The spiritual truth given to the church at Ephesus was not Ephesus-specific, though the reasons for why that truth was given were location-specific. I hope you can understand the difference, here. It's the difference between what was given and why it was given. The Ephesians needed to be told certain truths applicable to all for reasons that were (at least somewhat) unique to the believers in Ephesus.