A.T. Robertson Commentary 1 Corinthians 10:1

"For I would not, brethren, have you ignorant, that our fathers were all under the cloud, and all passed through the sea;" — 1 Corinthians 10:1 (ASV)

For (γαρ). Correct text, not δε. Paul appeals to the experience of the Israelites in the wilderness in confirmation of his statement concerning himself in 9:26f. and as a powerful warning to the Corinthians who may be tempted to flirt with the idolatrous practices of their neighbours. It is a real, not an imaginary peril.

All under the cloud (παντες υπο την νεφελην). They all marched under the pillar of cloud by day (Exodus 13:21; Exodus 14:19) which covered the host (Numbers 14:14; Psalms 95:39). This mystic cloud was the symbol of the presence of the Lord with the people.