"in the assertion that" is correct and can be used in written English.
It is typically used to introduce a statement or claim that is being made. Here is an example: "In the assertion that all humans have equal rights, the speaker emphasizes the importance of treating everyone with fairness and dignity."
The crux of Remonstrant Arminianism lay in the assertion that human dignity requires an unimpaired freedom of the will.
The voice-over, in the assertion that a person is "a riddle to himself," points toward Malick's overarching idea.
The artwork she has produced is grounded in the assertion that traditionally feminine qualities emotionality, bodiliness, domesticity are worthy subjects, and also means, for art making.
Finn, my nine-year-old son, was more interested in the assertion that this was the cave where Odysseus was captured in by Polyphemus the cyclops.
Critics will revel in the assertion that James choked or froze in the fourth quarters of this compelling series, whatever that means.
Gary Fereday London There is nothing stunningly original in the assertion that communism and socialism have much in common with religion.
"Who, in view of this fact, will call in question the assertion that this is a Bible nation?
In addition, the assertion that some rules of international armed conflicts are also applicable in NIACs is becoming more commonplace (Columbia Law School Human Rights Institute, cited in Cleveland 2014).
Rowling, who lives in Scotland, questioned the assertion that there were no anti-English feelings whatsoever in the Scottish National Party.
In social psychology, the assertion that most social cognition and social behavior is automatic in nature, executed outside of conscious awareness and conscious control.
The doctrine in question amounts to the assertion that, in the aftermath of a financial crisis, banks must be bailed out but the general public must pay the price.
