{While Peter yet spake} (eti lalountos tou Petrou).
Genitive absolute of present participle, still going on. {The
Holy Ghost fell} (epepesen to pneuma to hagion). Second aorist
active indicative of epipipt(935c), old verb to fall upon, to
recline, to come upon. Used of the Holy Spirit in  8:16  10:44  11:15 . It appears that Peter was interrupted in his sermon by
this remarkable event. The Jews had received the Holy Spirit
( 2:4 ), the Samaritans ( 8:17 ), and now Gentiles. But on this
occasion it was before baptism, as was apparently true in Paul's
case ( 9:17f. ). In  8:16  19:5  the hands of the apostles were
also placed after baptism on those who received the Holy Spirit.
Here it was unexpected by Peter and by Cornelius and was
indubitable proof of the conversion of these Gentiles who had
accepted Peter's message and had believed on Jesus Christ as
Saviour.