Healthy, strong members of the congregation believe that their own health, their own efforts, and their own good behavior keeps them out of hell. As Edwards puts it, such worshippers believe their safety and salvation comes from:
your healthy constitution, and your own care and prudence, and best contrivance, and all your righteousness
Edwards reminds them that the Bible says that every foot will slip and that they cannot stay out of hell just because they are healthy and strong. They are deluding themselves to believe they themselves can prevent disaster through their own efforts because they do not realize how precarious their situation is. Edwards states:
if God should withdraw his hand, they would avail no more to keep you from falling, than the thin air to hold up a person that is suspended in it.
God's grace alone saves the healthy from hell.
Edwards emphasizes that it is sincere acceptance of Jesus Christ's salvation that is necessary to protect the believer. Believers need to turn to Jesus, repent of their sins, and renew their relationship with Christ to avoid hell.
What makes the sermon so memorable and famous is the vivid imagery Edwards uses to describe the precarious situation of humans. He states that the healthy and strong, as well as the sick and frail, are passing through life on a wooden bridge that hangs suspended over hell. Below it, fire and brimstone burn brightly and the damned are roasted forever in the flames. We walk across that bridge throughout life, but without realizing how rickety and damaged it is. It is missing many planks, so that at any moment we could plunge through to hell. Many of its planks are rotten too and at any time could give under own weight and send us down into the fiery pit. We can only be safe (saved) because of God's love.
As Edwards make abundantly clear, believers, no matter how strong they may be, can be cast down into the fiery pit of hell just as quickly as anyone else. The same goes for those who are healthy. An individual's health, no more than his or her care, prudence, or righteousness, is enough to keep them out of hell. If God is angry with you, then you're going to hell, and there's nothing you can do about it.
What Edwards is challenging here is the notion that there's something believers can do to prevent themselves from being cast down into hell for all eternity. Evidently, some backsliders among Edwards's congregation have got in into their heads that their strength and good health are clear signs that they are part of the elect, predestined to go to heaven.
Edwards wants to disabuse anyone of such a belief. He takes great pains to emphasize that the decision to send sinners to hell is God's and God's alone and that therefore no one has the right to be so complacent as think that they're not one of the damned simply because they're strong and healthy.
In Edwards’ view, the members of the congregation he’s addressing believe that they will be saved from Hell by their own actions rather than by the grace of God. More specifically, they believe that simply living good and righteous lives is enough to spare them from God’s ultimate punishment. With this sermon, Edwards is trying to convince them that only what he refers to as “the great change of heart,” or a personal spiritual experience of God, will save them from His wrath, and that merely following the prescriptions of a religious life is no substitute for being genuinely born again:
How dreadful is the State of those that are daily and hourly in Danger of this great Wrath, and infinite Misery! But this is the dismal Case of every Soul in this Congregation, that has not been born again, however moral and strict, sober and religious they may otherwise be.
As to why they believe this, Edwards thinks it is due to their own vanity and arrogance, and a “falling away” from the true word of God; they trust more in their own judgment than in the actual words of scripture (or at least Edwards’ interpretation of it). This view was at the heart of the Great Awakening, an effort by Edwards and others to reverse what they saw as a sort of watering down of church doctrine that resulted in people observing the forms of religion without understanding what was at its core.
According to Edwards, healthy, strong members of the town foolishly believe their own strength and abilities can save them from God's wrath.
In Edwards's opinion, those who have yet to descend to the depths of hell are only momentarily safe because of God's mercy. He accuses the members of the town of trusting their own healthy constitutions to keep them out of the fires of hell. Perhaps members of the town believe in their relative safety because they have conceivably lived what they consider to be good lives: after all, they have refrained from self-indulgent habits and have pursued every righteous course known to them.
You are kept out of Hell, but don’t see the Hand of God in it, but look at other Things, as the good State of your bodily Constitution, your Care of your own Life, and the Means you use for your own Preservation. . . if God should let you go, you would immediately sink. . . plunge into the bottomless Gulf, and your healthy Constitution, and your own Care and Prudence, and best Contrivance, and all your Righteousness, would have no more Influence to uphold you and keep you out of Hell, than a Spider’s Web would have to stop a falling Rock.
Basically, in his sermon, Edwards argues the strong and healthy members of the town should rely less on their own contrivances than on God to keep them from the fires of hell.