The believing Jews knew the Holy Spirit had fallen on the Gentiles because they heard them speak with tongues and magnify God.
In Acts 10, the Holy Ghost fell on Cornelius and his household while Peter was still preaching. The Jewish believers who came with Peter were astonished because this gift was also poured out on Gentiles. Their proof is found in Acts 10:46: 'For they heard them speak with tongues, and magnify God.' Les Feldick teaches that God used this gift of other languages as the sign that convinced the Jewish witnesses.
Acts 10:44-46 — This is the direct passage where the Holy Ghost falls on Gentiles in Cornelius's house and the Jewish believers hear them speaking in tongues and magnifying God.
While Peter was still speaking, the Holy Ghost fell on all them which heard the word (Acts 10:44). The record says the Jewish believers who had come with Peter from Joppa were astonished because on the Gentiles also was poured out the gift of the Holy Ghost (Acts 10:45). Then Acts 10:46 states plainly what the Jewish believers heard: 'For they heard them speak with tongues, and magnify God.' The text presents this as the observable evidence that the Spirit had been given to the Gentiles.
Les Feldick sets the scene in Acts 10: Peter went to the house of the Gentile Cornelius because God forced him to go, despite it being unlawful for a Jew to keep company with another nation. Peter took six believing Jews with him from Joppa, making seven witnesses. As Peter preached about Jesus of Nazareth as Israel's Messiah, the Holy Spirit fell on all the Gentiles who heard the word -- before any water baptism. Les explains that God had to prove to Peter and the other six Jews that He was doing something totally new: saving Gentiles. The way they knew was the gift of languages: 'They heard them speak with tongues and magnify God.' According to Les, these were other languages, and this phenomenon stopped Peter's preaching in its tracks. The astonishing thing was that Gentile salvation had never happened in this way before.
Acts 10:46 says they heard the Gentiles 'speak with tongues, and magnify God.' Les Feldick points out that this was the God-given sign of languages; the tongues here were other languages, not a private or unknown speech. The audible result was that the Gentiles were magnifying God, and that is what the Jewish believers could recognize.
Peter went to Cornelius's house with six believing Jews from Joppa, for a total of seven. Les emphasizes that these Jews were believers themselves, but they were astonished because on the Gentiles also was poured out the gift of the Holy Spirit. This was something that had never happened before in the Jewish experience; salvation was all Jewish up to that point, and a Gentile receiving the Spirit in this way was shocking to them.
Les Feldick teaches that God had to prove to Peter and the six other Jewish men that He was doing something totally new -- saving Gentiles. The Spirit fell 'while Peter yet spake' (Acts 10:44), before any baptism, and the accompanying sign of languages confirmed that God had indeed poured out the Spirit on Gentile believers. Peter later recalled this at Jerusalem as evidence that God could save Gentiles without making them proselytes of Judaism.
Peter's Journey to Cornelius's House
Les explains that God compelled Peter to go to the house of a Gentile, setting the stage for the Holy Spirit to fall on Cornelius and his household.
The Jews Heard Them Speak With Tongues
Les answers the question directly: the Jewish believers knew because they heard the Gentile believers speak with tongues and magnify God.
The Gift of Languages as the Sign
Les says God used the same phenomenon of speaking in other languages to let the astonished Jews know the Holy Spirit had been poured out on the Gentiles.
The Six Jewish Witnesses
Les highlights that Peter brought six believing Jews with him, making seven witnesses who were astonished to see the Spirit fall on Gentiles.
The Spirit Fell Before Baptism
Les shows that the Holy Spirit fell while Peter was still speaking and before water baptism, proving that God was doing a totally new work of saving Gentiles.
Gentile Salvation Was an Astonishment
Les stresses that up to this point salvation was Jewish, so the pouring out of the Spirit on Gentiles was an astonishment to the Jewish believers.
The Holy Spirit Fell on All Who Heard
Les describes the moment: while Peter preached to Cornelius's Gentile household, the Holy Spirit fell on all those who heard the word.
Peter's Later Testimony at Jerusalem
Les recalls that Peter's eyes were opened to the fact that God could save Gentiles without making them proselytes of Judaism.
Acts 10:44: While Peter yet spake these words, the Holy Ghost fell on all them which heard the word.
This shows the Spirit fell suddenly during Peter's message, before any baptism.
Acts 10:45: And they of the circumcision which believed were astonished, as many as came with Peter, because that on the Gentiles also was poured out the gift of the Holy Ghost.
This records the astonishment of the Jewish believers who witnessed the event.
Acts 10:46: For they heard them speak with tongues, and magnify God.
This is the specific evidence that the Jewish witnesses heard the Gentiles speaking in other languages and praising God.
They heard the Gentile believers speak with tongues and magnify God (Acts 10:46). Les Feldick says this gift of other languages was the God-given sign that let the Jewish witnesses know the Spirit had come upon them.
The Holy Ghost fell while Peter was still speaking, before the water baptism that followed in Acts 10:48. Les Feldick uses this timing to show that God was saving these Gentiles directly.
They were astonished because on the Gentiles also was poured out the gift of the Holy Ghost (Acts 10:45). Les Feldick explains that up to that point salvation had been Jewish, and this was something that had never happened before.
Watch the Teaching: What the Bible Really Says About Speaking in Tongues