How do we know the Church at Corinth was a true gospel church?

The authenticity of the Church at Corinth is evident in Paul's address to them. He outlines that they were 'sanctified in Christ Jesus' and acknowledges the grace and enrichment they had received from Him. Paul's high commendations affirm that the Corinthians stood firmly in the faith and possessed spiritual gifts, thus validating their status as a true church. However, it is crucial to understand that true churches can still experience disarray and sinful practices, reflecting the ongoing struggle between the flesh and the Spirit within its members.