The Westminster Shorter Catechism begins with the question ...

Notice that Leviticus 19:17 says, “You shall not hate your brother in your heart,” and verse 18 prohibits holding a grudge and requires loving others—all clearly matters of the heart. It is true that far more of the Old Testament deals with externals than the New Testament, but God has always required externals to flow from a heart of love for Him and for our neighbor. ................
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