“lapis lydius: Lydian stone, touchstone, standard, or benchmark;
originally a hard black flint used by the ancients to test the purity of gold and silver according to the streaks left on the stone when rubbed by the metals, by extension a standard and invariable test of quality. Scripture is thus frequently referred to as lapis lydius.”Richard Muller, Dictionary of Latin and Greek Theological Terms: Drawn Principally from Protestant Scholasticism, Term: lapis lydius.
Scripture is the touchstone, the standard, the black flint upon which all theological teaching is tested and found genuine or wanting. Scripture is also the touchstone of Scripture being a first principle. Only God’s words can properly verify God’s words so also Scripture, being God’s words, can only be properly verified by Scripture itself through the living testimony of the Spirit of God.
As the Scriptures teach in I Cor. 2:11, “For what man knoweth the things of a man, save the spirit of man which is in him? even so the things of God knoweth no man, but the Spirit of God.” In other words, not only is Scripture the touchstone of all theological claims like Jesus was born of the virgin Mary and I John 5:7 is not Scripture, but Scripture is also its own touchstone and therefore able to answer whether 1 John 5:7 is actually Scripture.