'Testify Concerning' in the Bible

He came to be a witness, so that he might testify concerning that Light, so that all people would believe [in Jesus] through him [i.e., through his testimony. See Acts 19:4].

He himself was not that Light, but came [only] to testify concerning that Light.

You search through the Scriptures [i.e., the Old Testament], because you think that you will [learn how to] have never ending life by [reading] them. And these Scriptures are what testify concerning me.