Being filled with the Holy Ghost, also referenced as the Holy Spirit in some Bible translations, was a New Testament apostolic anointing that all the believers of Jesus Christ were called to walk in. From the time Jesus ascended to heaven after His resurrection until His glorious return, His Ghost (Spirit) was intended to dwell within His saints to empower them to be witnesses of His resurrection and His return.
The apostles taught that the prophets of old had the Spirit of Christ in them and spoke of the sufferings of Jesus and the resurrection to come. Those who prophesied spoke by the Spirit of Christ in hope but would not get to see the resurrection manifest like New Testament believers would. (1 Peter 1:10-12) The Holy Ghost is another measure of that same Spirit that was given to believers of the New to help them experience the resurrection of Christ in real-time. It was a spiritual, personal experience with Jesus Christ because it was the resurrected Christ in them.
The whole point of the gospel after Jesus' death was His resurrection from the dead and the infilling of the Holy Ghost, or Himself, within His people so that the gospel's message could continue in them and manifest on the earth. When the Holy Ghost is in full effect, the news of His resurrection will grow among His people. The message is Jesus is Alive!
The Holy Ghost event in Acts 2 on the Day of Pentecost was Jesus adding power to the witness of the Spirit that His saints already possessed. Peter, for example, already had a witness that Jesus was the Christ, the Son of God, but after he partook of the Holy Ghost, he didn't just have a witness; he became the witness of the Son of God and the Spirit of Christ. His spirit became the ghost of Jesus.
God's saints are meant to be His ghost on the earth until He returns.
Question: If I already have the Spirit of Christ in my heart, why do I need the Holy Ghost? As I stated in the example with Peter, the difference is in you being taught by the Spirit versus you doing the teaching by the Spirit. It's a matter of student versus teacher and which one you are. Having the Spirit of Jesus in your heart makes you His, but having the Holy Ghost makes you the witness of His resurrection. The Holy Ghost makes you the witness. The Spirit of Christ speaks of ownership, while the Holy Ghost speaks of empowerment to be a Christ.
Jesus defines what the Holy Ghost is for.
Using the term ghost may seem strange, but that's what a Spirit of Christ is when it abides in you, and you become a new man, a new creature created in Christ Jesus. You take on His likeness, which is His heavenly resurrected image. The physical body is a means to an end; its corruption limits it. But those constraints do not limit the Spirit of Christ in the power of resurrection. A heavenly ghost (Spirit) can do all things through Christ. (Philippians 4:13)
Acts 1:8 But you will receive power when the Holy Ghost has come upon you, and you will be MY witnesses in Jerusalem and in all Judea and Samaria, and to the end of the earth.
The Holy Ghost makes you a true witness of Jesus after His resurrection. You are a witness of the heavenly because Jesus has opened heaven, and the Holy Ghost has revealed heaven to you. In the NT, after the apostles received the Holy Ghost, they diligently began to call Jesus the Christ (the Anointed One) instead of the prophesied Messiah. This is indicative (a sign) of their Jesus experience.
It also allowed them to be partakers with Him as the anointed ones; therefore, they preached their gospel through the lens of Jesus Christ. Because of the power of the Holy Ghost, they became the anointed Christ on the earth to witness to everyone through many signs and wonders.
Who is the Holy Ghost?
The secret to understanding the Holy Ghost is understanding His part in God's Spirit hierarchy. God is one Spirit, yet there are diversities to His Spirit that help accomplish specific administrations for God's purpose. The Holy Ghost has all to do with Jesus after His resurrection. He empowers you to tell God's story through the power of resurrection life. The story is Jesus is Alive! Whether through preaching, laying on of hands, prophecy, intercession, fellowship, or any other work of God, the Holy Spirit will always do the works of God from a resurrection mindset. This is why the outpouring of the Holy Ghost in Acts 2 was so important to the New Testament apostles and the Church. It allowed them, as well as us, to focus on Jesus in His resurrection.
The Holy Ghost works in all things and many different ways, but you cannot get distracted and only focus on one part. The purpose of the Holy Ghost is to give you your Jesus Christ resurrection experience. That's why the Holy Ghost is still relevant today to all believers. It wasn't just a Bible thing; it's a believers thing.
Acts 2:38-39 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 Ghost. For the promise is for you and for your children and for all who are far off, everyone whom the Lord our God calls to Himself."