24 Now a Jew named Apollos, a native of Alexandria, came to Ephesus. He was an eloquent man, competent in the Scriptures. 25 He had been instructed in the way of the Lord. And being fervent in spirit, he spoke and taught accurately the things concerning Jesus, though he knew only the baptism of John. 26 He began to speak boldly in the synagogue, but when Priscilla and Aquila heard him, they took him aside and explained to him the way of God more accurately. (Acts 18) [ESV]
Here the issue is clear. Apollos' knowledge is incomplete and so it is deficient. He knows only of John's baptism and is unaware of the instructions Jesus gave regarding the new baptism (Matthew 28:19). The implication is Apollos is failing to teach or is unaware of the resurrection:
because, if you confess with your mouth that Jesus is Lord and believe in your heart that God raised him from the dead, you will be saved. (Romans 10:9)
If Apollos failed to preach God raised Jesus from the dead, his message would be accurate and yet incomplete. In other words, it is possible to use the teachings Jesus gave before He was crucified to demonstrate He is the Christ without also teaching He was raised from the dead. For example the Sermon on the Mount is even today seen as significant to Christianity:
"The Sermon is the longest continuous discourse of Jesus found in the New Testament and has been one of the most widely quoted elements of the Canonical Gospels. It includes some of the best-known teachings of Jesus, such as the Beatitudes, and the widely recited Lord's Prayer. The Sermon on the Mount is generally considered to contain the central tenets of Christian discipleship."
Despite the significance of the Sermon, it lacks the message of the Resurrection. Teaching only the baptism of John and the teachings of Jesus as declared in the Sermon on the Mount would be an accurate description of Apollos.
Likewise those disciples Paul encountered in Ephesus knew only of John's baptism. This suggests they had an accurate and yet incomplete understanding of who Jesus is.
The simplest explanation is the message Philip first preached was incomplete:
4 Now those who were scattered went about preaching the word. 5 Philip went down to the city of Samaria and proclaimed to them the Christ. 6 And the crowds with one accord paid attention to what was being said by Philip, when they heard him and saw the signs that he did. 7 For unclean spirits, crying out with a loud voice, came out of many who had them, and many who were paralyzed or lame were healed. 8 So there was much joy in that city...12 But when they believed Philip as he preached good news about the kingdom of God and the name of Jesus Christ, they were baptized, both men and women. (Acts 8)
It is easy to presume Philip explicitly taught God raised Jesus from the dead as Peter did on the day of Pentecost (Acts 2:24-36). But, clearly something is not entirely right: despite being baptized, the Samaritans did not receive the Holy Spirit. The simplest explanation is Philip either assumed the Samaritans were aware of the Resurrection, or he emphasized other aspects of the message. Philip is associated with Stephen as one of the seven chosen by the church in Jerusalem to oversee the distribution of food. (Acts 6:1-6). Stephen's speech makes no mention of the Resurrection. It is possible, as Stephen shows, to use the Scriptures to prove who Jesus was without also preaching who Jesus is: the Risen LORD.
In the Fourth Gospel some Samaritans acknowledged Jesus was the Savior of the World while He was alive (John 4:42). Obviously that was accurate and would bring salvation. At the same time, they did not receive the Holy Spirit because Jesus had yet to be glorified (John 7:39). In a like manner preaching Jesus is the Savior of the World or even that He is the Lamb of God who takes away the sin of the world without including the message of the Resurrection is accurate yet incomplete: it fails to include His glorification. This would explain why some believers experienced a delay in receiving the Holy Spirit.
It is clear from the letter, the Ephesians were taught and believed God raised Jesus from the dead:
15 For this reason, because I have heard of your faith in the Lord Jesus and your love toward all the saints, 16 I do not cease to give thanks for you, remembering you in my prayers, 17 that the God of our Lord Jesus Christ, the Father of glory, may give you the Spirit of wisdom and of revelation in the knowledge of him, 18 having the eyes of your hearts enlightened, that you may know what is the hope to which he has called you, what are the riches of his glorious inheritance in the saints, 19 and what is the immeasurable greatness of his power toward us who believe, according to the working of his great might 20 that he worked in Christ when he raised him from the dead and seated him at his right hand in the heavenly places, 21 far above all rule and authority and power and dominion, and above every name that is named, not only in this age but also in the one to come. 22 And he put all things under his feet and gave him as head over all things to the church, 23 which is his body, the fullness of him who fills all in all. (Ephesians 1)
Paul's preaching exemplifies the fully developed Christian doctrine which includes not only the high standard of righteousness as found in the Sermon on the Mount but the Crucifixion and Resurrection of Jesus as Christ. It is the complete message which brings the Holy Spirit who enables the believer to live as a follower of Christ.
An accurate but incomplete belief is a deficiency which need not deny salvation, but reasonably explains the delay in receiving the Holy Spirit.
- Sermon on the Mount