Why does H.F. risk infection to go into the city?

The narrator's initial plan to leave the city was foiled by his servant deceiving him and leaving him alone though he had promised to help him flee. The narrator began wondering whether or not this was a sign that he ought to stay; perhaps it was the will of Heaven that he remain. God would no doubt preserve his health and safety if he followed God's will by staying, as opposed to acting contrary to God's intentions for him.

His brother, however, frightened him with stories about others who had acted similarly. The narrator once again felt like he should leave the city, especially as he noticed rates of infection around him rising rapidly.

One evening H.F. walked home in the isolated streets and tried to "resolve first, what was my Duty to do, and I stated the Arguments with which my Brother had press'd me to go into the Country, and I set against them the strong Impressions which I had on my Mind for staying..." He cried out for the Lord to direct him, and to his surprise, he happened to land on a page of the Bible that he had been flipping through that spoke of plague and pestilence and the Lord's protection for those who trusted in Him. From that moment on, the narrator resolved to remain in London and trust in God to protect him and deliver him from the plague.