All quotations are from the King James Version of the Holy Bible. What the Lord Jesus said in John 7:37-39 was as follows; "In the last day, that great day of the feast, Jesus stood and cried, saying, If any man thirst, let him come unto me, and drink. He that believeth on me, as the scripture hath said, out of his belly shall flow rivers of living water. (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.)" What a promise!
He was speaking about the baptism of the Holy Ghost that happened on the Day of Pentecost. Listen to the description of that day, recorded an Acts 2:1-4, "And when the day of Pentecost was fully come, they were all with one accord in one place. And suddenly there came a sound from heaven as of a rushing mighty wind, and it filled all the house where they were sitting. And there appeared unto them cloven tongues like as of fire, and it sat upon each of them. And they were all filled with the Holy Ghost, and began to speak with other tongues, as the Spirit gave them utterance." What a fulfillment of the promise!
The Bible says of God in Malachi 3:6a, "For I am the LORD, I change not...." and in Hebrews 13:8, "Jesus Christ the same yesterday, and to day, and for ever." Because God doesn't change and because the Lord Jesus is the same today as He was yesterday, and will be forever, what He fulfilled on the day of Pentecost is still available for you and I today. Friend, if you have never experienced the fulfillment of the promise of the Father, (See Acts 1:8), let me encourage you to ask God to fulfill His promise to you today.