The phrase "powerful testimony" refers to a very compelling and influential account or statement given by someone about an experience or belief they have. Full definition
It's the most powerful testimony of anyone I have ever heard who struggled with this particular issue and came to have 100 % complete confidence in his eternal salvation. (redeeminggod.com)
It gave powerful testimony to a widespread longing for freedom,.... (firstthings.com)
It is hard to believe that so famous and so powerful a testimony to the emotional intensity of which friendships are capable could have been excluded from this anthology by accident or ignorance. (firstthings.com)