What did the Corinthians turn from before they began to worship the one true God?

The Corinthians turned from idols to the living God, coming out of abject paganism with its immorality and idolatrous practices. They left those practices behind and began to worship the one true God, becoming changed people who were living epistles.

Les explains that the Corinthians, like the Thessalonians, had turned from idols to the living God. They had come out of abject paganism with all of its immorality and idolatrous practices and had begun to worship the one true God. He notes that Paul calls them his epistle, changed people who were living epistles written in the heart, known and read of all men. Les says the same is true of many who come to know the Lord through simple teaching—they become living epistles. He then cites Ephesians 4, saying believers are manifestly declared to be the Epistle of Christ, ministered by us, written not with ink but with the Holy Spirit, not on tables of stone, but as living epistles with the Spirit's work written on the heart.

1 Corinthians 1:1

Paul, called to be an apostle of Jesus Christ through the will of God, and Sosthenes our brother,

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What had the Corinthians turned away from and turned to before Paul wrote to them?

Before Paul wrote to them, the Corinthians had turned from idols to the living God. They had come out of abject paganism, with its immorality and idolatrous practices, and had begun to worship the one true God. Their changed lives served as a living epistle.