"relentless insistence" is a correct and usable phrase in written English.
It is typically used to describe someone's persistent and unwavering demand or stance on something. Example: Despite multiple rejections, John's relentless insistence on getting the job ultimately paid off when the company finally offered him a position.
Hewitt drilled away at the Federer backhand with a relentless insistence, hoping the cracks would show.
At the same time, their relentless insistence on the positive clearly drives him nuts.
The only difference with Mr. Bradley is his relentless insistence that he's different.
MR. SHILLER stands very nearly alone in his relentless insistence that markets move on feelings and herd behavior, not corporate fundamentals.
The noble Confucian model of the morally grounded intellectual speaking truth to power in a single dramatic confrontation was called on so often as to become, seemingly by intention, an unnoble and relentless insistence.
As recession and banking crises have enveloped Greece, Ireland, Portugal, Spain and Italy, the crisis response, led by Germany, has been dominated by a relentless insistence on self-defeating austerity and piecemeal rescue plans.
There was relentless pressure.
The challenge is to be relentless in our insistence in calling out lies and misinformation.
They were relentless in their insistence on discovering who they were, what drove them to do what they had done and, in the end, to make amends.
The women in my group were relentless in their insistence on discovering who they were, what drove them to do what they had done and, in the end, to make amends.
So relentless was her insistence that Houston had a problem that the star eventually let slip the demure mask perfected on her road to multiplatinum crossover status and pulled a metaphoric razor from her wig.
