Steadfastness comes from Allah, who makes firm the hearts of the believers: this idea runs through these verses. They mention the believers made firm in battle, the Messenger ﷺ strengthened in the face of the disbelievers, the revelation sent down to make firm those who believe, and what consolidates faith in the soul: spending in the way of Allah and bearing the hardship of obedience.
Verses gathered from a topical index of the Quran, in Mushaf order; every verse is quoted in full.
And if We had decreed upon them, "Kill yourselves" or "Leave your homes," they would not have done it, except for a few of them. But if they had done what they were instructed, it would have been better for them and a firmer position [for them in faith].