The HS falling upon Cornelius had nothing to do with his personal salvation. It was water baptism that was commanded to Cornelius, verse 47.48 and that command alone made water baptism necessary.
In the context;
1) God sent an angel to Cornelius to tell him to send for Peter
2) God gave Peter a vision telling Peter the Gentiles were "clean"
3) God baptized the Gentiles with the HS.
God did these things to prove to the Jews that salvation was not meant for the Jews only but also for the Gentiles.
In Acts 11:4-14 Peter told the Jews in Jerusalem about these things God did and the result was " When they (Jews) heard these things, they held their peace, and glorified God, saying, Then hath God also to the Gentiles granted repentance unto life." Acts 11:18 So baptism with the HS was what God used to prove to the Jews that salvation was not just meant for the Jews only but also for the Gentiles and it had nothing to do with Cornelius' personal salvation.
In Acts 15:11 Peter said Jews and Gentiles are "saved in like manner". The "like manner way" the Jews in Acts 2:38 and Gentiles in Acts 10:47,48 were saved was water baptism in the name of the Lord for remission of sins, not baptism with the HS.
Since God was proving to the Jews that salvation was not for them only but also for the Gentiles, that is why Peter said "Can any man forbid water, that these should not be baptized, which have received the Holy Ghost as well as we? And he commanded them to be baptized in the name of the Lord. Then prayed they him to tarry certain days."
Forbidding water baptism to the Gentiles would have gone against God's will in salvation being brought to the Gentiles. Thereby the Gentiles salvation was through water baptism and no Jew was to forbid that from happening lest they find themselves fighting against God's will.
1 Sam 19:20ff Saul and his messengers were not saved men but the spirit of God came upon them and they prophesied.