To answer your good question, let’s read John 7:37-39, “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.)”

It will greatly help if we know the context of the Lord’s words in verse 38.  Verse 37 tells us that it was “in the last day, that great day of the feast” that the Lord made this statement.  What feast is being spoken of here?  Verse 2 tells us that “the Jews’ feast of tabernacles was at hand.”  There were seven ‘set feasts’ that the Lord’s people were to observe (Lev. 23; Numbers 29:39).  To understand about the Feast of Tabernacles, let’s read Leviticus 23:34-36, “Speak unto the children of Israel, saying, The fifteenth day of this seventh month shall be the feast of tabernacles for seven days unto the LORD. On the first day shall be an holy convocation: ye shall do no servile work therein. Seven days ye shall offer an offering made by fire unto the LORD: on the eighth day shall be an holy convocation unto you; and ye shall offer an offering made by fire unto the LORD: it is a solemn assembly; and ye shall do no servile work therein.”  The Israelites were instructed in verses 42-43, “Ye shall dwell in booths seven days; all that are Israelites born shall dwell in booths: That your generations may know that I made the children of Israel to dwell in booths, when I brought them out of the land of Egypt: I am the LORD your God.”  The Hebrew word for ‘tabernacles’ in verse 34 is the same Hebrew word used for ‘booths’ in verse 42.  This feast was to commemorate the days when the Lord brought His people out of Egypt and they lived in tents (booths).  This feast also looked forward to the Millennial day when the Lord’s earthly people will enter into the fullness of earthly blessings.

“Though not mentioned in the OT, the Jews had a ceremony of carrying water from the Pool of Siloam and pouring it into a silver basin by the altar of burnt offering for each of the first seven days of the Feast of Tabernacles.  On the eighth day, this was not done, which made Christ’s offer of the water of eternal live even more startling.  The Jewish people had gone through this religious observance, and yet their hearts were not satisfied because they had not truly understood the deep meaning of the feast.  Just before they departed for their home, on the last day, that great day of the feast Jesus stood and cried out to them.  He invited them to come to Him for spiritual satisfaction.  Pay particular attention to the words.  His invitation was extended to anyone.  His gospel was a universal gospel.  There was no one who could not be saved if he would simply come to Christ.” (William MacDonald).

So, on this eighth day, when the people were ‘naturally thirsty’, the Lord offers to satisfy their ‘spiritual thirsts’.  His invitation, ‘if any man thirst’ shows that He was speaking to those who realized their need.  Indeed, it is ONLY those who realize their need of salvation that will come to the Lord to be saved.  This is akin to the Lord’s words in Matthew 11:28, “Come unto me, all ye that labour and are heavy laden, and I will give you rest.”  It is to those who realized their weariness in their struggle with sin and the weight of their sins that burdened them that the Lord invited to come to Him for rest. 

To those who came to Him, the Lord promised that, “out of his belly shall flow rivers of living water.”  The meaning of this saying is explained in verse 39, “(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.)”  This is the promise of the indwelling Spirit to all those who put their faith in the Lord Jesus Christ.  The Spirit would not come to indwell every believer while the Lord was still on the earth.  The Lord would later tell His disciples in John 16:7, “Nevertheless I tell you the truth; It is expedient for you that I go away: for if I go not away, the Comforter will not come unto you; but if I depart, I will send him unto you.”  The Spirit descended upon all believers on the day of Pentecost (Acts 2:1-4).  Now, He continues to dwell in every true believer as we read in Ephesians 1:13, “In whom ye also trusted, after that ye heard the word of truth, the gospel of your salvation: in whom also after that ye believed, YE WERE SEALED WITH THAT HOLY SPIRIT OF PROMISE.”

Have you come to the Lord Jesus to have your spiritual thirst satisfied?  Have you come to Him, realizing your need, and put your faith in Him as your savior?  In John 4:14 the Lord said, “But whosoever drinketh of the water that I shall give him shall never thirst; but the water that I shall give him shall be in him a well of water springing up into everlasting life.”  (393.6)