Question? Was Saul a believer before the filling of the Holy Spirit on the road to Damascus.
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Acts 9:17 So Ananias went and entered the house. He laid his hands on Saul1 and said, "Brother Saul, the Lord Jesus, who appeared to you on your way here, has sent me so that you may regain your sight and be filled with the Holy Spirit." (Act 9:17 NRS)
Were all the people Peter was preaching to believers?
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Acts 10:44 While Peter was still speaking, the Holy Spirit fell upon all who heard the word. (Act 10:44 NRS)
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Acts 10:46 for they heard them speaking in tongues and extolling God. Then Peter said, (Act 10:46 NRS)
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Acts 10:47 "Can anyone withhold the water for baptizing these people who have received the Holy Spirit just as we have?" (Act 10:47 NRS)
NRS Acts 11:15 And as I began to speak, the Holy Spirit fell upon them just as it had upon us at the beginning.
(Act 11:15 NRS)
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Acts 11:16 And I remembered the word of the Lord, how he had said, 'John baptized with water, but you will be baptized with the Holy Spirit.' (Act 11:16 NRS)
We must allow God to give the blessing of the Holy Spirit when and as He will.
NRS Hebrews 2:4 while God added his testimony by signs and wonders and various miracles, and by gifts of the Holy Spirit, distributed according to his will.
(Heb 2:4 NRS)
This may not satisfy you but I personally also have witnessed the filling of the Holy Spirit on both believers and non-believers.