1 Corinthians 9:2

9:2 If I am not an apostle to others, at least I am to you, for you are the confirming sign ¹  of my apostleship in the Lord.

1 Corinthians 14:22

14:22 So then, tongues are a sign not for believers but for unbelievers. Prophecy, however, is not for unbelievers but for believers.

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¹ tn Grk “the seal.”