The simple, straight forward answer to this is that here Paul is reasserting that Jesus was, is and always will be GOD and he is teaching the Corinthians they are not to be overly confident just because they are Christians because they can fall from grace. 

Having said that lets go back and do a little digging to scripturally backup that view.

First, we need to recall Exodus 17:6 and Numbers 20:1.  In both those passages God, through Moses brought water from a rock to quinch the thirst of the Israelites in the desert. 

This, I believe, is what Paul is referencing when he wrote, “all drank the same spiritual drink.”  It was spiritual in the sense that God provided it from a source from which one would not normally get water, a rock just as they – the Israelites – were eating “spiritual food” (I Corinthians 10:3) when they received their food – the manna – from heaven.   

Next we need to remember, Jesus compared His teaching to water of life, that is spiritual life, in John 4:14 – “… but whoever drinks of the water that I shall give him shall never thirst; but the water that I shall give him shall become in him a well of water springing up to eternal life.” 

Also remember, He compared Himself to the Bread of Life in John 6:33 to 51 – “Our fathers ate the manna in the wilderness … Truly, truly, I say to you, it is not Moses who has given you the bread out of heaven, but it is My Father who gives you the true bread out of heaven.  For the bread of God is that which comes down out of heaven, and gives life to the world. … I am the bread of life; he who comes to Me shall not hunger, and he who believes in Me shall never thirst. … I am the bread of life.  Your fathers ate the manna in the wilderness, and they died.  This is the bread which comes down out of heaven, so that one may eat of it and not die.  I am the living bread that came down out of heaven; if anyone eats of this bread, he shall live forever; and the bread also which I shall give for the life of the world is My flesh.” 

Then concerning the phrase “the rock was Christ” we need to look at Deuteronomy 32, particularly verse 4. 

Deuteronomy 32 is a song by Moses when Joshua had been appointed to take his – Moses’ place – (Deuteronomy 31:1 to 8).  In Deuteronomy 31:9 to 13 Moses provided instruction concerning the Reading of the Law an in verses 14 to 29 Joshua is commissioned. 

Moses then sings the song found in Deuteronomy 32 in which, in verses 3 & 4 he sings, “For I proclaim the name of the LORD; Ascribe greatness to our God!  The Rock! His work is perfect, For all His ways are just; A God of faithfulness and without injustice, Righteous and upright is He.” 

Later in the song, in verses 15 and 18 God is once again compared to a Rock in two parallel metaphors: “Then he forsook God who made him, And scorned the Rock of his salvation” in verse 15 and then “You neglected the Rock who begot you, And forgot the God who gave you birth” in verse 18. 

Then concerning verses 30 to 33, one commentator said: “When God was with his people, there would be great victory. But in the rhetorical question of Deu_32:30, the situation is imagined in reverse. Israel’s armies would be pursued by one and put to flight by two. The rhetorical question implies that the only way such a situation could happen would be if God (their Rock) had delivered his people into the enemies’ power. Since neither the enemy nor the enemy gods (their rock, Deu_32:31) were in themselves sufficiently powerful to defeat the God of Israel, the defeat of God’s people had to be interpreted as having happened with the permission of God. Even our enemies may be our judges (Deu_32:31) – the principle, which recurs in the later prophets, is made clear that the enemies of the people of God could be instrumental in the execution of God’s judgment. 

The substance of Deu_32:32-33 refers to the enemies of God’s people, who would be instrumental in the execution of divine judgment. Employing the metaphor of a vineyard, its grapes, and the product of wine, the poet declares that the enemies are basically corrupt, having their roots in Sodom and Gomorrah, places associated with evil since ancient times (see also Deu_29:23). Given the corruption of Israel’s enemies, God’s use of them could in no way reflect some merit inherent in the enemies, but it did indicate the ignominy Israel brought upon itself by its sinful behavior.”  (Craigie, Peter C. The Book of Deuteronomy, The New Internation Commentary on the Old Testament) 

See also, 2Samuel 22:47 – The LORD lives, and blessed be my rock; And exalted be God, the rock of my salvation … 

Psalms 18:46 – The LORD lives, and blessed be my rock; And exalted be the God of my salvation … 

Psalms 89:26 – He will cry to Me, 'Thou art my Father, My God, and the rock of my salvation.' 

Psalms 95:1 – O Come, let us sing for joy to the LORD; Let us shout joyfully to the rock of our salvation. 

AND many other passages in the Old Testament reference and show God as a Rock. 

Two other passages we need to consider before we move on from this phrase. 

The first is John 1:1 & 2, “ In the beginning was the Word, and the Word was with God, and the Word was God.  He was in the beginning with God.” 

The second is John 8:58 where Jesus told some Jews who questioned Him, “Truly, truly, I say to you, before Abraham was born, I am.” 

Jesus always has been, is now, and always will be GOD. 

Therefore, I Corinthians 10:1 to 4 when Paul is recounting how the Israelites were brought through the Red Sea (baptized into Moses) and ate and drank spiritual food – the manna and water provided by God from the Rock – that JESUS, CHRIST, was there and He was part of that Rock known in the Old Testament as God or, “the LORD.” 

Paul then sets the tone for the next portion of his letter when he says in I Corinthians 10:5, “Nevertheless, with most of them God was not well-pleased …” 

Then, in verse 6 he explains WHY he is recounting all these things, “… these things happened as examples for us …” 

Then in verse 7 and 8 Paul tells the Corinthian brethren, “… do not be idolaters … Nor let us act immorally …” 

Remember, some if not most of the Corinthian brethren had been converted out of paganism and idol worship was prominent in Corinth and part of that pagan, idol worship involved “religious” prostitution. 

Verse 8 is a reference back to Numbers 25:1 to 9 where, according to Moses 24,000 fell from the plague because, Numbers 25:3, “Israel joined themselves to Baal of Peor, and the LORD was angry against Israel.”  Moses in Numbers said 24,000 while Paul in I Corinthians said 23,000.  DO NOT let this become a stumbling block because both men were using round numbers: anything between 23,000 and 24,000 could be characterized by either number. 

Finally, Paul draws this portion of his letter to a conclusion in I Corinthians 10:11 to 13, “Now these things happened to them as an example, and they were written for our instruction, upon whom the ends of the ages have come.  Therefore let him who thinks he stands take heed lest he fall.  No temptation has overtaken you but such as is common to man; and God is faithful, who will not allow you to be tempted beyond what you are able, but with the temptation will provide the way of escape also, that you may be able to endure it.” 

In other words, learn from what happened to the Israelites.  Just because God was leading them, providing them with spiritual food and water, and guiding them did not mean that they were therefore, “fool proof.”  They could and did fall at times and when they did they suffered the consequences. 

Just so, you Corinthians, just because you have been converted over to Jesus Christ does not mean you are “fool proof.”  You too can fall from grace (Galatians 5:4 – You have been severed from Christ, you who are seeking to be justified by law; you have fallen from grace.) and when you do you need to repent and be restored, James 5:16 and I John 1:9.