Affirm and Testify

Affirm verb - To state as a fact usually forcefully.

Testify is a synonym for affirm in speech topic. In some cases you can use "Testify" instead a verb "Affirm", when it comes to topics like declare, say, declare the truth of something.

Testify verb - To make a solemn declaration under oath for the purpose of establishing a fact.

Affirm is a synonym for testify in say topic. You can use "Affirm" instead a verb "Testify", if it concerns topics such as assert, attest, vouch for.