The phrase "assertion that he believed" is correct and usable in written English.
It can be used when discussing someone's claim or statement regarding their beliefs or thoughts.
Example: "The assertion that he believed in the importance of honesty was evident in his actions."
Alternatives: "claim that he believed" or "statement that he believed".
The company contended that the convictions challenged the credibility of Mr. Boeken's assertion that he believed smoking was safe.
Prosecutors wrote that they believed Mr. Harris's defense would center upon the assertion that he believed he was being permitted by the police to walk on the roadway, but added that they thought old Twitter messages might show otherwise.
The most incendiary bit of his testimony was his assertion that he believed his phone was hacked by the Mail on Sunday, which, in February, 2007, ran a story suggesting, as Grant put it, that "my relationship with my then-girlfriend was on the rocks because of my persistent late-night flirtatious phone calls with a plummy-voiced studio executive from Warner Brothers".
His love of myths and his devout faith came together in his assertion that he believed mythology to be the divine echo of "the Truth".
The genesis for Guevara's assertions relied on the fact that he believed the example of the Cuban Revolution was "something spiritual that would transcend all borders".
Though that investigation has not been completed, Mr. Rumsfeld said today that he believed Mr. White's assertion that he had not been aware of the talking points.
To the assertion that he is a socialist: "I believe deeply that the free market is the greatest force for economic progress in human history".
They added that "multiple senior officials have stated that U.S. intelligence agencies do not collect information or dossiers on 'millions of Americans,' " assertions that they believe have been contradicted by revelations of Internet data mining and a court order asking a Verizon subsidiary to turn over logs of telephone calls.
Despite his neglect of church attendance, and frequent assertions that he didn't believe in God, he feared hellfire and damnation ("a very superstitious Catholic," observed Elspeth) and his lifelong engagement with the moral drama of Catholic theology, wrote Fraser, made him "a religious poet in a secular age".
The government has said that it stands by him and believes his assertion that he had done nothing wrong.
