"truly testify" is correct and usable in written English.
You can use it to indicate the truthfulness of a statement or event, often in a legal context. For example, "The witness was asked to truly testify about what they saw that day in court."
So someone who bears true witness against someone who has not wronged him (e.g., witness A who truly testifies that he saw B murder C) wrongs the person against whom he bears witness, and therefore acts both justly and unjustly.
(Full disclosure: Yours truly testified of her history with mild asthma and lung surgery to correct a rare lung disorder needed while living close to the Valmont plant).
She's been the wackiest serial TV guest since Monica Lewinsky's lawyer, William Ginsburg, turning up everywhere except "Hollywood Squares" to insist that she would really, truly love to testify before the 9/11 commission but wouldn't -- except in private.
I suggested that maybe Congress invite the truly poor to testify before them and just listen.
In his decision this week, Judge Kaplan made it clear that he wanted to explore further whether the witness's decision to testify was truly voluntary.
The large ledgers on display testify to some truly impressive customers over the years, including Oscar Wilde (who failed to pay off his bill) and Winston Churchill, whose 10-a-day cigar habit, in his view, contributed to his long-lasting health (he lived to 90).
Only Zimmerman and Martin truly know the answer; one refused to testify, the other couldn't.
But Middlebrooks insisted that Wright was always willing to testify if the committee members truly wanted her to do so.
When it's done correctly, bodysurfing, as in channelling the energy of the wave with your body rather than a surfboard or bodyboard, is truly graceful, as a quick browse on YouTube will testify.
Now in the Hague to testify in the Charles Taylor case, Farrow finds herself well and truly back in the spotlight.
