Testimony

In law and in religion, testimony is a formal statement that something is true. The words "testimony" and "testify" both come from the Latin word "testis", which refers to a third-party witness.
Law.
In the law, testimony is a kind of evidence. It comes from a witness who says that something is true. Testimony may be spoken or written. It is usually made by oath or affirmation under penalty of perjury (which is a crime that somebody can be charged with if they give testimony that is not true).
Religion.
Some Christians use the term "to testify" or "to give one's testimony" to mean "to tell the story of how one became a Christian".