Christians are identified as being equally and completely (in the past tense) members of God's church (1 Corinthians 12:13). Everybody who receives Jesus Christ as Lord and Savior possesses the Holy Spirit (Romans 8:9; Ephesians 1:13-14). Our bodies are the temple of the Holy Spirit (1 Corinthians 6:16-19). The example of Cornelius serves as a perfect illustration of believers simultaneously receiving salvation and the Holy Spirit (Acts 10:43-48). The baptism of the Spirit is not separated from the moment of conversion in Scripture.
The handful of episodes of the Holy Spirit falling on people who were already followers of God in the Book of Acts are a unique historical event. They took place during an important transition of salvation history. Christ had ascended into heaven and was glorified by the Father. The scope of redemption was expanded to the Gentiles. What we see described in biblical history is not to be treated as a prescriptive standard for doctrine. Moreover, what happened at Pentecost was a fulfillment of Joel 2:28-32. These people had the fullness of the Holy Spirit, but He chose a different mode of operation at this point.