We can read more words written to the church in Corinth than any other group of Christians, 15,318 words over the two letters, compared to the next highest which is Romans at 9467, and the next after that is only 3,102 to the Galatians (source). Paul had a lot to say “to those sanctified in Christ Jesus” in that ridiculously sinful first-century city.
In his greeting to those Corinthian saints, even knowing the many and profitable reproofs and corrections they needed, Paul could still say that he gave thanks to God for God’s grace given to them, and he could see that God had given them gifts for serving each other. For all their problems, Paul trusted that God would sustain them to the end, sanctifying them for the Lord’s return. Their faithfulness depended on God’s loyal and steadfast love.
God is faithful, by whom you were called into the fellowship of his Son, Jesus Christ our Lord. (1 Corinthians 1:9 ESV)
God “called” these believers. This is the effectual call, the internal call. It’s more than an invitation (typically referred to as external call), it’s the transforming of heart, mind, and will along with an enabling to call on the name of the Lord. John Calvin described it as follows:
“[God’s] calling, by which He draws us to himself, is not like that of a man, who tries to persuade someone by words alone, but is a secret power, which by the working of the Spirit penetrates into the innermost recesses of the soul, and affects it by its own efficacy.” (Institutes III.xi.5)
And God called these believers “into fellowship,” fellowship with Himself and His Son (by His Spirit). This fellowship is divine, this fellowship discourages division among the brothers (see the very next paragraph in 1 Corinthians 1:10ff), this fellowship is the aim of God’s call, God’s faithfulness that calls the unloved to be the beloved. We are His beloved gathered by Him to share communion with Him and with Christ’s body.