1 Corinthians 10:4

And did all drink the same spiritual drink: for they drank of that spiritual Rock that followed them: and that Rock was Christ (KJV)

Then: As Baker (p. 142) and Wright (p. 125) explains the spiritual drink for Old Testament Israelites was the water which flowed from the rock as Moses struck it - twice. Traditions infer that the rock went with the Israelites as they travelled. As they all drank the same physical water, the Lord reassured the Israelites that he was with them. Jesus, who is the source of living water, assured Christians that he was with them. The Rock of the Old Testament is the Christ of the New (Baker, p. 143). The metaphor made good sense to the Corinthians and enforced Paul's teachings about ways to live and stay under the promise of God's love.

Now: There is no controversy or contention about today's Christ followers being part of God's plan from the time of creation. Paul's teachings were the same as those of the other apostles. Jesus is the Christ, the rock upon which all stand and the way of salvation.

Extra source: Cornerstone Biblical Commentary 1 Corinthians (William Baker), 2 Corinthians (Ralph Martin & Carl Toney), ed. by Philip Comfort. Carol Stream: Tyndale House Publishers, c2006.