The phrase "a steadfastness which" is correct and usable in written English.
It can be used when describing a quality of unwavering determination or loyalty in a person or situation.
Example: "Her steadfastness which inspired the team to persevere through difficult times was commendable."
Alternatives: "a determination that" or "an unwavering commitment that".
The steadfastness with which she turned the newspaper into a powerhouse was most visible during the turbulent 1970's, when The Post and The New York Times joined in a confrontation with the federal government over publishing the Pentagon Papers, a secret study of the Vietnam War, and in The Post's dogged pursuit of the Watergate scandal that brought down President Richard M. Nixon.
Abed took to the streets in 2011, participating in the pivotal Friday of Rage and Friday of Steadfastness protests, which helped start the insurrection against the dictator, Bashar al-Assad.
Mr. Goldstein will be remembered for the integrity, compassion, vitality, steadfastness with which he lived life and the boundless love, loyalty and generosity he showered upon his family, friends and clients.
Whether they vote with us or against us, the other nations see the actions we take and notice the steadfastness with which we fight for our beliefs, and for the good of mankind.
In those tournaments, the Americans galvanized a nation with a steadfastness that was both heart-stopping and nerve-racking.
Playing right field with an enlarged prostate requires a certain singularity of purpose, a steadfastness of mind.
You'll find more periods in his lines of dialogue than question marks, indicating a steadfastness of thought.
Sushenya remains unwilling to compromise his principles to the end, a steadfastness that will bring the film its disquieting final scene.
In Petrarch, the white doe wears a collar studded with diamonds and topazes (emblems of steadfastness and chastity), which proclaims her untouchable: "It has pleased my Caesar to make me free".
