The Holy Spirit came up on Cornelius "while he(Peter) was still speaking". You can't be a Christian without belief, and you can't have belief without hearing the Word of God. Peter was not done teaching the Gospel when Cornelius started speaking in tongues.
You know ... that they hadn't heard enough ?
I know Peter had just started to speak. I also know that Peter said that the Holy Spirit coming upon Cornelius was like what happened to him and the other Apostles at Pentecost. Was that "baptism" of the Holy Spirit for salvation? No. Neither was this.