Those who believe in faith alone insist that because of Acts 11:17 they received the Holy Spirit by believing on Jesus they were saved. How must one defend against it?
In Acts 11, Peter is defending his actions in Acts 10.
"But Peter explained it to them in order from the beginning, saying: "I was in the city of Joppa praying; and in a trance I saw a vision, an object descending like a great sheet, let down from heaven by four corners; and it came to me. When I observed it intently and considered, I saw four-footed animals of the earth, wild beasts, creeping things, and birds of the air. And I heard a voice saying to me, 'Rise, Peter; kill and eat.' But I said, 'Not so, Lord! For nothing common or unclean has at any time entered my mouth.' But the voice answered me again from heaven, 'What God has cleansed you must not call common.' Now this was done three times, and all were drawn up again into heaven. At that very moment, three men stood before the house where I was, having been sent to me from Caesarea. Then the Spirit told me to go with them, doubting nothing. Moreover these six brethren accompanied me, and we entered the man's house. And he told us how he had seen an angel standing in his house, who said to him, 'Send men to Joppa, and call for Simon whose surname is Peter, 'who will tell you words by which you and all your household will be saved.' And as I began to speak, the Holy Spirit fell upon them, as upon us at the beginning. Then I remembered the word of the Lord, how He said, 'John indeed baptized with water, but you shall be baptized with the Holy Spirit.' If therefore God gave them the same gift as He gave us when we believed on the Lord Jesus Christ, who was I that I could withstand God?" When they heard these things they became silent; and they glorified God, saying, "Then God has also granted to the Gentiles repentance to life" (Acts 11:4-18).
What gift is Peter referring to when he said that God gave them the same gift as He gave the apostles? Peter said it was the same gift that fell on the apostles at the beginning. He is referring to the gift of speaking in other languages -- speaking in tongues. "While Peter was still speaking these words, the Holy Spirit fell upon all those who heard the word. And those of the circumcision who believed were astonished, as many as came with Peter, because the gift of the Holy Spirit had been poured out on the Gentiles also. For they heard them speak with tongues and magnify God" (Acts 10:44-45). The apostles received the same gift at the start of the church. "And they were all filled with the Holy Spirit and began to speak with other tongues, as the Spirit gave them utterance. ... And when this sound occurred, the multitude came together, and were confused, because everyone heard them speak in his own language. Then they were all amazed and marveled, saying to one another, "Look, are not all these who speak Galileans? And how is it that we hear, each in our own language in which we were born?" (Acts 2:4, 6-8)
Because of this event, Peter said he could not withstand God. "Then Peter answered, "Can anyone forbid water, that these should not be baptized who have received the Holy Spirit just as we have?" And he commanded them to be baptized in the name of the Lord" (Acts 10:46-48).
This was an unusual occurrence. It did not normally happen when a person believed. Peter said that when he saw it happened with Cornelius' household, he was reminded of Jesus' words and what had happened to him and the rest of the apostles. If it really was the gift of salvation that Peter was referring to, then no one was saved between Acts 2 and Acts 10.
Then take note of the result. Because of seeing God's gift of tongues come upon Cornelius' household, Peter commanded them to be baptized. Why? The Jewish Christians answered this in Acts 11:18, "Then God has also granted to the Gentiles repentance to life." One talks about baptizing in water, the other talks about repentance to life. Reminds me of Acts 2:38, "Then Peter said to them, "Repent, and let every one of you be baptized in the name of Jesus Christ for the remission of sins ..."
For more details, see: When Was Cornelius Saved?