It used to be called Whitsuntide, and it used to be a public holiday. The UK Government did away with the name and they did away with the holiday. But they can't do away with its significance. Tomorrow, 49 days after Easter Sunday, is Pentecost Sunday, when we remember how those early Christians were filled with the Holy Spirit.
On the evening of the day Jesus rose from the dead, He appeared to His disciples. Then the disciples were glad when they saw the Lord (John 20:20). Thomas wasn't there on that occasion, but he was there a week later.
Then he said to Thomas, "Reach your finger here, and look at my hands; and reach your hand here, and put it into my side. Do not be unbelieving, but believing."
And Thomas answered and said to him, "My Lord and my God!"
Jesus said to him, "Thomas, because you have seen me, you have believed" (vv27 - 29).
What does the Bible say are the things necessary for salvation? If you confess with your mouth the Lord Jesus and believe in your heart that God has raised him from the dead, you will be saved (Rom 10:9). Can anyone doubt that those disciples were believers at that point?
But they did not yet have all that they needed. They were to tarry in the city of Jerusalem until they were endued with power from on high (Luke 24:49), for, said Jesus, you shall be baptized with the Holy Spirit not many days from now (Acts 1:5).
They waited from the days of the Jewish feast of Passover, when Jesus died and rose again, to the Jewish feast of Shavuot, or Pentecost. And when the day of Pentecost had fully come. . . and they were all in one place. . . there came a sound from heaven, as of a rushing mighty wind. . . and forked tongues, as of fire. . . and they were all filled with the Holy Spirit (Acts 2:1 - 4). Frightened believers were transformed into believers ablaze for God.
Peter explained that the promise of the Holy Spirit is to you and to your children, and to all who are afar off, as many as the Lord our God will call (v39) - not just to the disciples there at that time, but to future generations, to people in other places, to every one who is called to God's salvation.
Some say that you have everything when you are born again, regenerated, come to faith in Christ. Don't believe that.
There's more. Oh yes, there's more!