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God established a covenant with the nation of Israel in the Old Testament, and a covenant with his Church in the New Testament. Not all of the laws that applied to Israel apply to the Church, but all of the truths in the laws apply.

Here, in Matt. 18:16, Jesus himself is taking a truth from the old law and transforming it for the Church. He quotes Deut 19:15 which lays out some judicial instructions for Israel. The Church doesn’t have a court system, but justice still applies and is upheld in a Spirit-led way.