“ 19:1 And it happened that while Apollos was at Corinth, Paul passed through the inland  country and came to Ephesus. There he found some disciples. 2 And he said to them, “Did you receive the Holy Spirit when you believed?” And they said, “No, we have not even heard that there is a Holy Spirit.” 3 And he said, “Into what then were you baptized?” They said, “Into John's baptism.” 4 And Paul said, “John baptized with the baptism of repentance, telling the people to believe in the one who was to come after him, that is, Jesus.” 5 On hearing this, they were baptized in  the name of the Lord Jesus. 6 And when Paul had laid his hands on them, the Holy Spirit came on them, and they began speaking in tongues and prophesying. 7 There were about twelve men in all.” (Acts 19:1-7 (ESV)
It seems Apollos was not the only one to be a disciple or Jesus, and yet only have the baptism of John. Paul runs into some disciples like this in Ephesus, as it seems Apollos and Paul pass each other changing places. These disciples have not even heard of the Holy Spirit, and because of this Paul knows that they have not been baptized. Christian baptism comes with the Holy Spirit.
However, we see here that baptism is not an optional thing in the life of the Christian. One who believes cannot refuse to be baptized. One who believes will want to be baptized.