The main definitions of testimony are (1) a declaration of truth or fact, (2) a declaration by a witness under oath, and (3) a public declaration regarding a religious experience. Testament, in addition to meaning (1) something that serves as proof or evidence, may also mean (2) a statement of belief, (3) one of the two main divisions of the Bible, and (4) a written document providing for the disposition of a person’s property. So testament makes more sense in constructions like, “His kindness is testament to the fact that he is a good man.”
Still, testimony is so often used in place of testament that most dictionaries now list something that serves as proof or evidence as an accepted definition of testimony. But that doesn’t change the fact that testament is more conventional for this purpose.
Examples
Testament
It’s a testament to the generosity of Mercer’s residents and its volunteer organizations that such altruism survives. [NJ.com]
The average term for a franchisee is 20 years – a testament to the loyalty McDonald’s seeks to encourage. [Financial Times]
His home office, full of advanced accounting, management and business books, is testament to just how hard he’s worked. [Stuff.co.nz]
Testimony
The lengthy testimony that followed concerned almost every piece of evidence taken from the lodge. [CNN]
Earlier, court heard testimony from the teen’s co-accused admitting he stole the victim’s purse and wallet. [Calgary Herald]