To answer your good question, let’s read Acts 19:1-5, “It happened that while Apollos was at Corinth, Paul passed through the upper country and came to Ephesus, and found some disciples. He said to them, “Did you receive the Holy Spirit when you believed?” And they said to him, “No, we have not even heard whether there is a Holy Spirit.”  And he said, “Into what then were you baptized?” And they said, “Into John’s baptism.”  Paul said, “John baptized with the baptism of repentance, telling the people to believe in Him who was coming after him, that is, in Jesus.” When they heard this, they were baptized in the name of the Lord Jesus.” (NASB)

In this portion, we find the Apostle Paul in the city of Ephesus. Ephesus was a city of the Roman province of Asia. While there, Paul met some disciples. As he began to question them as to their knowledge and experience of the Christian life, he found they were sadly lacking in both. It is likely that these were Asian Jews, who, having been at Jerusalem at one point, had heard the preaching of John the Baptist, and received his baptism, believing in the coming Christ, whom John had proclaimed. It appears that they had gotten no more teaching and did not realize that Christ had already come. Paul, perceiving this, asked them if they had received the Holy Ghost since they believed. All believers, Jew and Gentile alike, receive the indwelling of the Holy Spirit at the moment of salvation. Acts 10:45 tells us, “All the circumcised believers who came with Peter were amazed, because the gift of the Holy Spirit had been poured out on the Gentiles also.” Yet, to these men, the Holy Spirit was a mystery. They had no knowledge of Him. They only knew John’s baptism unto repentance.

This was a very unique period in church history, where Old Testament and New Testament saints were here at the same time. These disciples that Paul met would be considered Old Testament saints. They were faithful to the scriptures, following John the Baptist’s teaching concerning the coming Messiah. They had not put their faith in the Lord Jesus Christ because they had not heard of Him, however they were faithful to what they knew.

The faithfulness of these disciples was manifested in that when they heard of the Lord Jesus, they were immediately baptized in His name, thereby showing their faith in Him and obedience to His Word. Let’s read Paul’s words in Acts 13:24-26 and 38-39, “After John had proclaimed before His coming a baptism of repentance to all the people of Israel. And while John was completing his course, he kept saying, ‘What do you suppose that I am? I am not He. But behold, one is coming after me the sandals of whose feet I am not worthy to untie.’ “Brethren, sons of Abraham’s family, and those among you who fear God, to us the message of this salvation has been sent…Therefore let it be known to you, brethren, that through Him (Christ) forgiveness of sins is proclaimed to you, and through Him everyone who believes is freed from all things, from which you could not be freed through the Law of Moses.” Paul showed that John spoke of a coming savior, and that Jesus is that savior!

These disciples (rightfully called) were faithful to what they had heard. When they heard more, they were faithful to that. They were baptized in the name of the Lord Jesus to show they truly believed in Him in accordance to what we read in Romans 6:3-6, “Do you not know that all of us who have been baptized into Christ Jesus have been baptized into His death? Therefore we have been buried with Him through baptism into death, so that as Christ was raised from the dead through the glory of the Father, so we too might walk in newness of life. For if we have become united with Him in the likeness of His death, certainly we shall also be in the likeness of His resurrection, knowing this, that our old self was crucified with Him, in order that our body of sin might be done away with, so that we would no longer be slaves to sin.” This type of incident could not be repeated today. Christ has come, was crucified, was raised again and has ascended into Heaven. We have His completed Word and we are called to believe upon Him for salvation. (314.1)