And That Rock Was Christ

The Rock that followed the children of Israel when they were in the wilderness was Christ. When they thirsted, out of that Rock flowed water.

"Moreover, brethren, I would not that ye should be ignorant, how that all our fathers were under the cloud, and all passed through the sea; and were all baptized unto Moses in the cloud and in the sea; and did all eat the same spiritual meat; and did all drink the same spiritual drink: for they drank of that spiritual Rock that followed them: and that Rock was Christ." - 1 Corinthians 10:1-4

The Feast of Tabernacles (Sukkot) is a seven day feast observing the forty years the Israelites wandered in the wilderness dwelling in tents (tabernacles). When the Temple stood, starting with the second night of the feast, there was a grand Water Drawing Celebration. The last day of the feast is called Hoshana Rabbah, or Great Salvation.
It was on this last day of the feast that Jesus stood and prophesied of the pouring out of His Spirit, His Holy Ghost. The pouring out of the water from the rock in the wilderness was commemorated by pouring a cup of water taken from the Shiloach (Siloam - John 9:7) stream into a hole in the altar. (There were two holes in the alter: one for the wine that accompanied every sacrifice, the other one was reserved for the Sukkot water. 1John 5:6)

I imagine it was at the moment of the pouring out of the water that Jesus stood and cried out to the people.

"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.)" - John 7:37-39

It was on the Day of Pentecost, after Jesus’ death, burial, resurrection and then ascension to heaven, that He poured out the gift of His Holy Ghost.

Peter preached how we can receive this gift today.

"Then Peter said unto them, Repent, and be baptized every one of you in the name of Jesus Christ for the remission of sins, and ye shall receive the gift of the Holy Ghost. For the promise is unto you, and to your children, and to all that are afar off, even as many as the Lord our God shall call." - Acts 2:38-39