1 Corinthians 16:20

All the brethren greet you. Greet ye one another with an holy kiss (KJV)

Then: A normal way of greeting each other was by use of a kiss. In Paul's calling it "holy," he's lifts the importance of interpersonal greetings to that of the friendship between Christ and the church. Baker (p. 248) explains that a holy kiss was not part of church liturgy until the days of Justin (100 - 165 AD).

Now: We greet with handshakes and maybe a cheek-to-cheek hug. Actions change; sentiments don't.

1 Corinthians 16:19

The churches of Asia salute you. Aquila and Priscilla salute you much in the Lord, with the church that is in their house (KJV)

Then: Corinthians know that Paul is near the end of his remarks. These are extra greetings from people the Corinthians would know and not know. He specifically mentions Priscilla and Aquila because of their name recognition. By adding in the fact that these two have church meetings in their home, Paul brings in a bond of familiarty with the Corinthians who do the same thing.

Now: In closing personal letters, many writers will add notes of friendship and greetings from another ".... sends his love" or ".... asked to be remembered." The purpose is to keep the bonds of love and friendship strong and active. Now, of course, it's not a usual church-to-church kind of event. Communcations and church work have changed so much that greetings are personal between minister and minister, leader and leader. The rest of the church is slightly aware of membership in fellow churches, but rarely on a personal basis.

1 Corinthians 16:17

I am glad of the coming of Stephanas and Fortunatus and Achaicus: for that which was lacking on your part they have supplied (KJV)

Then: Corinthians probably did not read this as a criticism. Other translators soften the meaning to Paul's being grateful for the help these three have given him while others in Corinth were just unable to come. Travel was not an easy task in those days. Taking the effort to bring Paul a status report was an exceptional event (Baker, p. 247). The biblical record shows no other church doing the same thing.

Now: Sister congregations use newsletters, denominational journals, annual conferences, Bible college contacts, member visits to other churches to find out what's happening. They learn ideas as to what works and what doesn't.

Extra Source: Cornerstone Biblical Commentary 1 Corinthians (William Baker), 2 Corinthians (Ralph Martin & Carl Toney), ed. by Philip Comfort. Carol Stream: Tyndale House Publishers, c2006.