"relentless faith" is correct and usable in written English.
You can use it when discussing someone's unwavering faith or determination in a particular belief regardless of the obstacles they may face. For example, "He was determined to achieve his goals, and he showed relentless faith in his own abilities."
His folksy manner masked a keen legal mind, a fiercely competitive streak and a relentless faith in the efficacy of punishment.
Maybe I'm being hopelessly American with my relentless faith in the possibility for a better future.
It must be hard to reconcile that kind of relentless faith with the humbling reality that medicine sometimes fails.
I have heard them both tell a story like this, which is more than anything else, a story about passion, commitment, relentless faith and ambition.
The film repeatedly seeks to evoke that contradictory sense of connection and alienation, not only in fervent Christianity but also in Islam, and it examines how the relentless pursuit of ultimate faith drives people to violent acts that are public manifestations of self-mortification.
Their beliefs are their beliefs after all, but I don't often contemplate the afterlife in the presence of a group whose faith is so relentless.
I am forever touched by your faith, courage, and relentless pursuit to show people that love is the very best path to becoming more fully human.
Entrepreneurship requires such relentless perseverance and a belief and faith in your vision".
The violence, though diminishing, is relentless, ravaging a crossroads of peoples and faiths in the plains where Arab Iraq meets the Kurdish mountains.
The trio are confronted by relentless horrors — executions, pillage, starvation — which challenge their religious faith.
In fairness to Alberto Moreno, roundly criticised for his performance in Germany, he repaid Klopp's show of faith in keeping the same starting line-up by helping set a relentless tempo.
