Testify is to tell what you know, what you have seen, and what you know to be the truth. God has done great things for his people we should be willing to tell the world about all his goodness. No matter what God has done many great things for his people over, and over, and over again. God testify of his people check out even Moses his disobedient yet God inspired man to write about Mose. Apostle Paul and all his bad doings God yet testified of him and allowed him to write his churches to teach, train, and encourage.