Luke uses βαπτισθήσεσθε (NASB: “you will be baptized”) in Acts 1:5 and then uses ἐπλήσθησαν (“and they were filled”) in Acts 2:4. Is there a reason for the change in verbs used to describe the baptism related to the Holy Spirit?
The literal meaning of being filled with the spirit is obvious.
Luke 1:41 Elizabeth was filled with the Holy Spirit.
Luke 1:67 Zechariah was filled with the Holy Spirit.
Luke 1:15 John was filled with the Holy Spirit even before he was born.
Luke 4:1 Jesus was filled with the Holy Spirit.
Acts 4:8 Peter was filled with the Holy Spirit.
Acts 4:31 A group of believers were all filled with the Holy Spirit.
Baptism of the Holy Spirit was first prophesied by John in Mt 3:11.
The act was described only twice. The first occasion was announced by Jesus in Acts 1:5 before he was taken up. It happened at the Pentecost (Acts 2:4). As pointed out by the OP, it was announced as an act of βαπτισθήσεσθε but when it actually happened, it was an act of ἐπλήσθησαν.
The second occasion occurred when Peter was at Cornelius’ House.
Acts 10:44 While Peter was still speaking these words, the Holy Spirit fell on all who heard the message.
Here the verb is fell and not filled. I don't think it is profitable to make a Federal case of the distinction between fell and filled here.
Verb - Aorist Indicative Active - 3rd Person Singular
Strong's Greek 1968: To fall upon, press upon, light upon, come over. From epi and pipto; to embrace or seize.
Later in Acts 11:16, Peter recounted the act as being baptized (βαπτισθήσεσθε) in the Holy Spirit.
Luke uses the term filled in Acts 2:4 to show that baptism of the Spirit requires filling but not vice versa. Similarly, Peter uses the term baptized in Acts 11:16 to ensure the the readers that there is no doubt what happened at Cornelius’ House is a baptism of the Holy Spirit and not just filling.
Not every filling of the Holy Spirit is a baptism of the Spirit. Baptism of the Spirit is special as Luke and Peter wanted to make it clear.
I think one is the idiomatic conclusion (Acts 2:4) to the other (Acts 1:5). In fact, John the Baptist had also prophesied that Jesus would baptise with the Holy Spirit (John 1:33, Matt 3:11, Luke 3:16, etc), and Acts 2:4 is the fulfilment of that prophecy and of Jesus in Acts 1:5.
The verb in Acts 1:5 is βαπτίζω (baptizó - to dip in water in order to wash or cleanse) closely related to βάπτω (baptó) - to dip a cup/vessel/cloth in liquid or dye in order to fill it.
Thus, we see some common Hebrew/Jewish idiom that well illustrates the ceremonial function of Christian baptism: Jesus told the disciples that they would be baptised in the Holy Spirit (Acts 1:5), and later on the day of Pentecost, they were filled with the Holy Spirit (Acts 2:4).
This illustrates what the rest of the New Testament also teaches; that the rite of baptism is (or should be, among other things) a symbol of the new Christian's reception of the Holy Spirit, Matt 3:11, Mark 1:8, Luke 3:16, Acts 1:5, 2:38, 8:12-16, 10:47, 48, 11:16, 19:4, 5. For example:
- John 7:37-39, "If anyone is thirsty, let him come to Me and drink. 38Whoever believes in Me, as the Scripture has said: ‘Streams of living water will flow from within him.’” He was speaking about the Spirit, whom those who believed in Him were later to receive." Thus, Jesus used water as a symbol of the Holy Spirit.
- Jesus also used a similar metaphor in John 3:5 saying, "Very truly I tell you, no one can enter the kingdom of God unless they are born of water and the Spirit."
- Acts 1:5, … you will be baptized with the Holy Spirit
- Acts 11:16, … you will be baptized with the Holy Spirit
- Note Rom 8:9 about life in the Spirit - "You, however, are controlled not by the flesh, but by the Spirit, if the Spirit of God lives in you. And if anyone does not have the Spirit of Christ, he does not belong to Christ." The beginning of such a life for the Christian is baptism when the old life is dead and buried.
In Matt 3:11, Mark 1:8, Luke 3:16 we find that the Holy Spirit makes His presence known at Jesus' baptism. Thus, while Rom 6 says that baptism is a symbol of our death and burial to the old way of life, it also represents our "resurrection" to a new life in the spirit (Rom 8:9, 10).