The phrase "adamantly state that" is correct and usable in written English.
It can be used when someone is expressing a strong and firm opinion or belief about something.
Example: "The scientist adamantly states that climate change is a pressing issue that must be addressed immediately."
Alternatives: "firmly assert that" or "strongly declare that".
After adamantly stating that he would not quit, Mr. Cohen changed his mind.
Today, Twitter's co-founder Biz Stone responded to the lawsuit, adamantly stating that Twitter will not settle with La Russa, calling the lawsuit an "unnecessary waste of judicial resources bordering on frivolous".
Right after that it adamantly states that this product is not detergent; it's soap.
Besides, Leno has adamantly stated that he sees his job as purely business, and not personal or political.
He was very good natured about the protest and adamantly stated that "all Petland puppies come from U.S.D.A. approved breeders".
She began the protest march by lighting a torch at her father's grave site and then passing it on to her spiritual mentor and the march organizer, Bishop Eddie Long of New Birth Missionary Baptist Church, who proudly carried the lit torch in a two-mile march through Atlanta with thousands of followers, Rev. Bernice King adamantly stated that the march was not against LGBTQ people.
Suddenly, I wanted to propose to him but felt somewhat silly about it after adamantly stating my views that I did not want an engagement.
States that have adamantly refused to expand should relent and take the generous federal funds.
Director Mendes adamantly denied this, stating that it was "just not true" and that Morgan's script treatment had been discarded once Mendes agreed to direct.
However, AnonOps (Anonymous Operations), the group's mouthpiece and network through which members frequently communicate, has adamantly stated via its news wing that it was not responsible for the outage.
