1 Corinthians 14:3 explains that prophecy edifies believers, which contrasts with tongues as a sign for unbelievers in this verse.

1 Corinthians 14:2 describes tongues as unintelligible speech to God, explaining why they function as a sign to unbelievers—they hear a foreign language.

1 Corinthians 14:19 prioritizes intelligible words over tongues in church, aligning with Paul's distinction that tongues are for unbelievers, not believers.

1 Corinthians 12:7 says all spiritual gifts are for the common good, and this verse specifies that tongues serve as a sign for unbelievers.

Acts 2:6-12 shows tongues as a sign to unbelieving Jews at Pentecost, illustrating Paul's point that tongues are for unbelievers.

Mark 16:17 lists tongues as a sign accompanying believers, while Paul says tongues are a sign for unbelievers—different emphasis on audience.

1 Thessalonians 5:20 commands believers not to despise prophecies — reinforcing that prophecy is for believers, as stated here.