For the prophecy came not in old time by the will of man: but holy men of God spake as they were moved by the Holy Ghost, 2nd Peter 1:21.
There can be no doubt that the Holy Ghost worked in the affairs of men during the Old Testament. He did so in a clandestine manner. Search as you will, but you will only find two distinct different persons who speak as God in the Old Testament. Verses such as; And God said, Let us make man in our image, Genesis 1:26, or Isaiah 57:15 in which the Lord says he dwells with him that is of a contrite and humble spirit make it clear that there are two different persons who are the same deity.
What is never clear in the Old Testament is that there is a third person who has worked behind the scenes to give scripture and to speak through the prophets. God's Spirit is mentioned often, but always appears to be the Spirit of one of the other two members of the Godhead. The person, the Holy Ghost is introduced by name in the Book of Matthew.
We learn in the New Testament that he is as much a distinct person in his own right as Jehovah God and as the Father. We also learn that he had been continually active throughout God's dealing with men. What changes with time is his relationship to men.
Think of a small town in which a boy and a girl both live, but have no real interactions with each other. If the boy's father hired the girl as a house maid, she would retain her exact same identity, but her relationship to the boy would change. If the boy was to fall in love with her and marry her, the relationship would change again.
There are transitions in the Holy Ghost's interactions with men. Prior to the resurrection of Jesus Christ, the Holy Ghost was present and working in men, but was not given to men in the sense that he is today. (But this spake he of the Spirit, which they that believe on him should receive: for the Holy Ghost was not yet given; because that Jesus was not yet glorified.), John 7:39.
After his resurrection, Jesus Christ breathed the Holy Ghost into his disciples to give them a one on one relationship with the Holy Ghost; And when he had said this, he breathed on them, and saith unto them, Receive ye the Holy Ghost, John 20:22. This giving was selective and limited to those few to whom the Lord would give it. He told them to tarry at Jerusalem for the promise of the Father. For John truly baptized with water; but ye shall be baptized with the Holy Ghost not many days hence, Acts 1:5.
At Pentecost the disciples were all immersed in the Holy Ghost. They shared a collective baptism in the Holy Ghost which joined their individual spirits with the spirit of Jesus Christ. That placed them in the literal physical resurrected body of Jesus Christ where it sits next to the Father; But he that is joined unto the Lord is one spirit, 1st Corinthians 6:17. For we are members of his body, of his flesh, and of his bones, Ephesians 5:30.
Don't confound the physical body of Jesus Christ which sits in heaven and which every person regenerated since Pentecost is part of by thinking that is the Protestant concept of some spiritual universal cloud called his body or the church. Jesus Christ is come in the flesh and if you have been redeemed by his blood, you have been made one with him in that body of flesh. That was done by the baptism of the Holy Ghost who is freely given at salvation.
By misunderstanding transitions, heretics love to jump on some facet of them to make doctrine. The analogy here would be the courtship of a boy and girl. Before any commitments were made, could they explore other relationships? Of course they could. When they were engaged to wed, was it moral for them to share a bed? Of course, it was not. Were either of them free to flirt or romance someone else? Of course. they were not. After they wed, they can share a marriage bed and they must stay committed one to another. Their relationship changed as they drew closer together and made commitments.
How foolish would it be for their children to take a snapshot of some aspect of their getting together and try to make rules for their lives if they ignored the transitions? They might enforce celibacy after marriage. They might allow other romantic entanglements after the engagement. By ignoring the orderly way in which a couple come together and meld into one, a person could wrestle some aspect of a relationship into bad doctrine for marriage. This is done all the time by people who ignore the orderly transition made by the Holy Ghost in his relationship with men. They take a snapshot of some aspect of the transition and try to make it doctrine for today.
There was a period of time after Pentecost wherein God used the apostles to get everyone on the same page. Those who were born again prior to Pentecost needed the laying on of an apostle's hands to receive the Holy Ghost. We see this in the case of 12 believing disciples who were converted under John the Baptist.
Acts 19:1 And it came to pass, that, while Apollos was at Corinth, Paul having passed through the upper coasts came to Ephesus: and finding certain disciples,
Acts 19:2 He said unto them, Have ye received the Holy Ghost since ye believed? And they said unto him, We have not so much as heard whether there be any Holy Ghost.
Act 19:3 And he said unto them, Unto what then were ye baptized? And they said, Unto John's baptism.
Acts 19:4 Then said Paul, John verily baptized with the baptism of repentance, saying unto the people, that they should believe on him which should come after him, that is, on Christ Jesus.
Acts 19:5 When they heard this, they were baptized in the name of the Lord Jesus.
Acts 19:6 And when Paul had laid his hands upon them, the Holy Ghost came on them; and they spake with tongues, and prophesied.
Such bestowing of gifts became part of the Apostle Paul's ministry. For I long to see you, that I may impart unto you some spiritual gift, to the end ye may be established, Romans 1:11. There is no one alive today who was saved prior to Pentecost and therefore that particular transition is over and done.
Then we have the case of the Samaritans who upon receiving the word of God at the preaching of Philip, needed Peter to lay hands on them (Acts 8:17). It was to Peter that Jesus Christ laid such heavy responsibility. And I will give unto thee the keys of the kingdom of heaven: and whatsoever thou shalt bind on earth shall be bound in heaven: and whatsoever thou shalt loose on earth shall be loosed in heaven, Matthew 16:19. Once Peter laid his hands upon the Gentiles and they received the Holy Ghost, from that time forward, Gentiles received the Holy Ghost upon conversion. Even Peter was shocked by that. 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, Acts 10:45.
The transition was complete. All too often, people see these events in the framework of dispensationalism wherein a whistle is blown and all the rules change. When a young believer asks a question about the transition times, all too often the answer is, "Oh, that's a different dispensation". What is needed are men of God who can explain the transitions between times in the bible by the events as they happened and by the promises of God. Explaining away transitions so cheaply is like telling a curious child that babies are delivered by a stork.
There will be a trump blown. There are many trumps in the bible and all too often those are confounded. When the camp of Israel moved they were to blow two silver (for redemption) trumpets.
Numbers 10:4 And if they blow but with one trumpet, then the princes, which are heads of the thousands of Israel, shall gather themselves unto thee.
Numbers 10:5 When ye blow an alarm, then the camps that lie on the east parts shall go forward.
Numbers 10:6 When ye blow an alarm the second time, then the camps that lie on the south side shall take their journey: they shall blow an alarm for their journeys.
We too shall move at the sound of the last trump. That will be the reverse of the Day of Pentecost. The Holy Ghost will be called back to heaven and every vessel wherein he is found will go with him. From that time on, those who are redeemed will receive the Holy Ghost on a one on one basis such as the Apostles did in John 20:22.