The scriptures are called testimonies in respect of God himself, because they give testimony to Him.
There is not a book of scripture but does testify of God and his attributes. In the book of Genesis, we have a testimony of his power in making the world, of his justice in drowning it, and of his goodness in saving it new. In the book of Exodus, we have a testimony of his providence in leading Israel out of Egypt, and over the Red Sea, and we have a testimony of his wisdom in giving them the Law. What should I name more?
In the New Testament – in the gospel – all is testimony. As the Old gave testimony to God, so the New to God in Christ. “To him gave all the prophets witness.” “These are they which testify of me.” Everywhere there is testimony to Christ: to his humility, in taking our nature; to his power, in working miracles; to his wisdom, in the parables that he spoke; to his patience and love, in the torments that he suffered for us.
Both Law and Gospel – the whole book of scripture in every part of it, and with regards to these things, is rightly called, “the testimonies of the Lord”.
(Richard Holdsworth.)