'Testify' in the Bible

who did testify the word of God, and the testimony of Jesus Christ, as many things also as he did see.

I Jesus have sent mine angel to testify unto you these things in the churches. I am the root and the offspring of David, and the bright and morning star.

For I testify unto every man that heareth the words of the prophecy of this book, If any man shall add unto these things, God shall add unto him the plagues that are written in this book: