We know the concept of mere Christianity.

In Acts, I ran across an example of mere John the Baptistry. In Acts 18, there are two different instances of the Lord and the Church acknowledging people outside its formal limits as already part of the fold in some sense.

First, the Lord tells Paul to linger in a city proselyting because “I have much people in this city.” He uses the present tense. These are unbaptized people who in many cases haven’t even heard of Christ but the Lord claims them. This is a startlingly latitudinarian ecumenicism. But it isn’t the shrug-your-shoulders kind of ecumenicism. The Lord tells Paul He has much people in the city because the Lord wants Paul to preach the gospel to them and baptize them.

Second, there is the missionary Jew named Apollos, who had been baptized by John the Baptist:

And a certain Jew named Apollos, born at Alexandria, an eloquent man, and mighty in the scriptures, came to Ephesus:
This man was instructed in the way of the Lord; and being fervent in the spirit, he spake and taught diligently the things of the Lord, knowing only the baptism of John.

A couple of the Christians hear him and take him aside to explain the parts of the gospel he is missing. That’s it.

There is an easy fluidity in the early Church that was probably necessary to its success, though it probably made the great apostasy inevitable also.

A couple of other interesting points from Acts 18:
* at one point the Holy Spirit “presses” Paul. We shouldn’t limit our conception of the Holy Ghost’s repertoire to intelligence and peace and joy. Remonstrance and command are also in there.
* Apollos’ education and ability to argue the case for Christ from the scriptures was said to be a great help to those who already “believed through grace.” There’s a lesson there about the relationship between belief and apologetics, faith and reason, and between what causes a conversion and what maintains the conversion.