The Book of Acts has some great material which helps in your argument:
Acts 2:37 Now when they heard this, they were cut to the heart, and they asked Peter and the other apostles, “What are we to do, my brothers?” 38 Peter [said] to them, “Repent and be baptized, every one of you, in the name of Jesus Christ for the forgiveness of your sins; and you will receive the gift of the holy Spirit. 39 For the promise is made to you and to your children and to all those far off, whomever the Lord our God will call.” 40 He testified with many other arguments, and was exhorting them, “Save yourselves from this corrupt generation.” 41 Those who accepted his message were baptized, and about three thousand persons were added that day.
Notice a couple things here. When they realized what they had done, and that they had sinned, and needed to turn to God, they ask Peter what they must DO. And Peter doesn’t respond with “accept Jesus as your Lord and personal Savior”. What does he say? Repent AND be baptized. And what would this repentance AND baptism do? It would give them forgiveness of sins. And notice, those who accepted the message had to be baptized.
*Acts 8:9 A man named Simon used to practice magic in the city and astounded the people of Samaria, claiming to be someone great. 10 All of them, from the least to the greatest, paid attention to him, saying, “This man is the ‘Power of God’ that is called ‘Great.’” 11 They paid attention to him because he had astounded them by his magic for a long time, 12 but once they began to believe Philip as he preached the good news about the kingdom of God and the name of Jesus Christ, **men and women alike were baptized. **13 Even Simon himself believed and, after being baptized, became devoted to Philip; and when he saw the signs and mighty deeds that were occurring, he was astounded.
14 Now when the apostles in Jerusalem heard that Samaria had accepted the word of God, they sent them Peter and John, 15 who went down and prayed for them, that they might receive the holy Spirit, 16 for it had not yet fallen upon any of them; they had only been baptized in the name of the Lord Jesus. 17 Then they laid hands on them and they received the holy Spirit.*
Notice here, that all of them believed, and were baptized. But notice, the Holy Spirit hadn’t come upon them yet. They were still lacking Confirmation (and Philip was a deacon, not a priest or bishop, so he could not confer the sacrament of Confirmation, that is why John and Peter went to them). But again, they BELIEVED, but still had not had the fullness of the Holy Spirit, that only came through the sacraments.
Now they will counter with Acts 10
Acts 10:42 He commissioned uso to preach to the people and testify that he is the one appointed by God as judge of the living and the dead. 43 To him all the prophets bear witness, that everyone who believes in him will receive forgiveness of sins through his name.” 44 While Peter was still speaking these things, the holy Spirit fell upon all who were listening to the word. 45 The circumcised believers who had accompanied Peter were astounded that the gift of the holy Spirit should have been poured out on the Gentiles also, 46 for they could hear them speaking in tongues and glorifying God. Then Peter responded, 47 “Can anyone withhold the water for baptizing these people, who have received the holy Spirit even as we have?” 48 He ordered them to be baptized in the name of Jesus Christ. 49 Then they invited him to stay for a few days.
But again, notice something here. First, God is not precluded from operating outside His own sacraments. Two, God was doing this out of the ordinary fashion because it was a declaration that the Faith is to given to the Gentiles as well. Finally, notice that they speak of baptism as something being withheld from them. It is NOT a symbol that shows their faith. Who is upset if a symbolic act is withheld?
Acts 19:1 While Apollos was in Corinth, Paul traveled through the interior of the country and came (down) to Ephesus where he found some disciples. 2 He said to them, “Did you receive the holy Spirit when you became believers?” They answered him, “We have never even heard that there is a holy Spirit.” 3He said, “How were you baptized?” They replied, “With the baptism of John.” 4 Paul then said, “John baptized with a baptism of repentance, telling the people to believe in the one who was to come after him, that is, in Jesus.” 5 When they heard this, they were baptized in the name of the Lord Jesus. 6 And when Paul laid [his] hands on them, the holy Spirit came upon them, and they spoke in tongues and prophesied. 7 Altogether there were about twelve men
Notice here, first, they were “disciples” already, but without full knowledge of the Holy Spirit. Second, the Holy Spirit didn’t come upon them until they were baptized AND confirmed (Paul laid his hands on them). So the sacraments confer the actions they claim.
Acts 22:12 “A certain Ananias, a devout observer of the law, and highly spoken of by all the Jews who lived there, 13 came to me and stood there and said, ‘Saul, my brother, regain your sight.’ And at that very moment I regained my sight and saw him. 14 Then he said, ‘The God of our ancestors designated you to know his will, to see the Righteous One, and to hear the sound of his voice; 15 for you will be his witness before all to what you have seen and heard. 16 Now, why delay? Get up and have yourself baptized and your sins washed away, calling upon his name.’
And finally, notice here, his sins are washed away through baptism.