"steadfast insistence" is a correct and usable phrase in written English.
It can be used when describing someone who is firmly and unwaveringly committed to their beliefs or demands. Example: Despite facing criticism and opposition, she maintained her steadfast insistence on fighting for equal rights for all.
Elsewhere, Nehring interrogates our steadfast insistence on balanced, healthy relationships, our readiness to condemn doomed, impossible entanglements.
The intent behind the gaming giant's steadfast insistence was understandable, if, perhaps, ill advised.
The Native peoples of California challenged and overcame colonial threats to this "moral management" through their steadfast insistence on continuing these land-based cultural practices despite who owns or claims ownership of these areas.
Gregory Craig's ouster as White House Counsel was, it appears, based on his steadfast insistence that the President adhere to his pledge to close Guantanamo, thereby running afoul of the political part of the Administration.
For their part, Wilpon and Katz are steadfast in their insistence that they had no hint that anything with Madoff was amiss.
Until Thursday, the government had been steadfast in its insistence on the plan's merits, saying it would bring fairness and transparency to an enormously costly set of perks.
But he was steadfast in his insistence that he had not been making any homophobic statements in the tweets, as gay rights groups have charged.
Pro-Brexit MPs told the paper that Merkel's comments marked "the beginning of a new realism" among the EU-27, who have hitherto been steadfast and united in their insistence that when the UK leaves the bloc, it will not be able to both curb EU immigration and keep enhanced access to the single market.
