A.T. Robertson Commentary 1 Corinthians 14:22

"Wherefore tongues are for a sign, not to them that believe, but to the unbelieving: but prophesying [is for a sign], not to the unbelieving, but to them that believe." — 1 Corinthians 14:22 (ASV)

For a sign (εις σημειον). Like the Hebrew and occasional Koine idiom also.