The spiritual drink provides the idea that there are "sacraments" in view: baptism and the bread & cup, or the Lord's Table. Paul indicates that the Israelites coming from Egypt were "baptized" (1 Cor 10:2) and subsequently ate the heavenly manna (bread) and spiritual drink (cup). Please note that manna is spiritual food according to Psalm 78:25, which is paired with spiritual drink in 1 Cor 10:4. However, these Israelites did not enter the Promised Land notwithstanding that they had been "sacramentalized" believers (that is, they had partaken of both baptism and the bread & cup). Paul is therefore drawing an analogy to the Hebrew Bible that sacraments (Baptism and the Lord's Table) in, of, and by themselves do not save; it is therefore wise that believers not assume or take their faith for granted (1 Cor 10:12). Even Peter insists that believers who believe that they are believers "be all the more diligent to make certain" of their salvation (2 Pet 1:10-11). Self-deception is the moral hazard as the following verse demonstrates.
Matthew 7:22-23 (NASB)
22 Many will say to Me on that day, ‘Lord, Lord, did we not prophesy in Your name, and in Your name cast out demons, and in Your name perform many [a]miracles?’ 23 And then I will declare to them, ‘I never knew you; depart from Me, you who practice lawlessness.’
An illustration will help to develop this idea: When the Israelites first left Egypt they in fact believed on the Lord (Ex 14:31). The Hebrew verb here for believing is the same verb used of Abraham, who was justified by faith. So the Israelites who escaped Egypt had believed by faith, were subsequently baptized into Moses in the cloud and sea (meaning that both Spirit and water had comprised the elements of the "baptism"), and even received the manna (bread) and spiritual drink (cup) from the Lord through the rock... but when that same faith was tested, their hearts were hardened by their own unbelief (Heb 3:19). That is, they had fallen away from faith (Heb 4:11) and were therefore denied entrance into the Promised Land.
In summary, the Christian New Testament portrays the believer on Jesus as one escaping the power of sin and death (like the Israelites who escaped Egypt through Moses). However, subsequent to the point of faith when they had believed (Ex 14:31), testing ensued in the wilderness (of life). If the faith takes root (compare to Matthew 13:1-23) that believer is saved and enters the Promised Land where more giants await. However, if that faith takes no root, the seed of faith perishes and the "believer" does not enter the Promised Land notwithstanding that such a person had been "sacramentalized" by people ordained by God.
Finally, and this point is very important, when the second-generation Israelites entered the Promised Land some forty years later, they partook of the sign ("sacrament") of circumcision at Gilgal according to Joshua 5:5-6. In other words, they were circumcised after crossing into the Promised Land, and not before. Thus faith and "sacraments" are complementary. Remember: the Israelites who did not enter the promised land were circumcised; were baptized into Moses; and partook of the manna (bread) and spiritual drink (cup) -- i.e., they were fully "sacramentalized" but were denied entrance into the Promised Land. Finally the word "Gilgal" in Hebrew (rolling away) means that circumcision at Gilgal had pointed to baptism, since according to Joshua 5:7-9 circumcision "rolls away" the reproach of Egypt (= imagery of sin and slavery). In other words, the stones from the Jordan River (site of baptism of John the Baptist who made apparent reference to the same stones in Luke 3:8) had remained at the side of the Jordan River while others were rolled to Gilgal as a commemoration of the circumcision of the second-generation Israelites, whose shame from Egypt "was rolled away" ("Gilgal") because of passage through the Jordan River -- the memorial stones make the correlation and provide the monument for future generations to ponder (Joshua 4:7). Thus circumcision as an adult (at Gilgal) or at birth (after eight days according to the Law of Moses) had pointed to the idea of water baptism later developed in the Christian New Testament.
In summary, the spiritual drink mentioned by Paul to the Corinthians was part of the larger imagery of the shadow of "sacraments" found in the Hebrew Bible, and their bearing on faith and salvation.