A testament is a statement of belief. The most famous testaments are the two parts of the Christian Bible: the Old and New Testaments.
A testament states a belief or gives some kind of direction. The document people leave behind at death is known as the "last will and testament." It gives lawyers directions for dividing their possessions among family and friends. Also, you could say that an A in math class is a testament to your math skills. If you give to charity, that’s a testament to your character. Any testament is a strong statement of some kind.