Spiritual fruit outwardly speaks of a godly heart.
Moreover, brethren, I declare unto you the gospel which I preached unto you, which also ye have received, and wherein ye stand; By which also ye are saved, if ye keep in memory what I preached unto you, unless ye have believed in vain. (1 Corinthians 15:1-2 KJV)
Paul is speaking in these scripture verses to those members of the Corinthian church who had openly declared or professed faith in the work of Jesus Christ on the cross for their own personal sin; along with taking away or covering the sin of all mankind, in His shed blood as a living sacrifice for sin.
The gospel of Jesus Christ; the sacrificial death, burial, and resurrection is a spiritual fact proven by many eye witnesses, including the apostles, of these ancient biblical times. This was the start of the earliest New Testament Churches whose members had witnessed first-hand His shed blood and payment for the debt of sin in full; marking the spiritual ledger or debt of all sin; past, present, and all future sin, and His resurrection unto new life.
Paul was reminding these Corinthian believers of the Truth of the gospel message. What it meant for them personally and collectively as the body of Christ who occupied His earthly church and in light of all eternity. This message from the mouth of Paul was inspired by God to be penned by a scribe into this book to the Corinthian believers and for our spiritual learning today as well.
Paul understood that some of these members who professed Christ had likely taken the gospel message in vain, professing with their mouth but not wholly believing it in their heart, never having personally accepted God as their Lord and Savior. False professions led to many serious doctrinal and relationship problems and errors within the Corinthian church. Paul was exhorting these “pseudo believers” to wake up from their slumber and coming alive spiritually alive unto new life in Christ fully understanding the gospel of Christ and becoming spiritual brethren and members of this Corinthian church and with God in Heaven.
False professions may fool those who are not spiritually perceptive fruit inspectors among us; but never fool God. God knows the heart of every individual; whether they are spiritually quickened alive or dead in their sins and trespasses.