The letter begins begins follows. It's the third and fourth paragraphs that I find especially moving.
My dear child,--The Works of Jonathan Edwards, Volume 16: Letters and Personal Writings (Yale University Press), 578-79
Before you will receive this letter, the matter will doubtless be determined, as to your having the smallpox. You will either be sick with that distemper, or will be past danger of having it. . . . But whether you are sick or well, like to die or like to live, I hope you are earnestly seeking your salvation. . . .
That which you met with, in your passage from New York to Newark, which was the occasion of your fever, was indeed a remarkable mine, a dispensation full of instruction, and a very loud call of God to you, to make haste and not to delay in the great business of religion. If you now have that distemper, which you have been threatened with, you are separated from your earthly friends; none of them must come to see you; and if you should die of it, you have already taken a final and everlasting leave of them while you are yet alive, not to have the comfort of their presence and immediate care, and never to see them again in the land of the living. And if you have escaped that distemper, it is by a remarkable providence that you are preserved.
And your having been so exposed to it, must certainly be a loud call of God, not to trust in earthly friends, or anything here below. . . . [T]his providence remarkably teaches you the need of a better friend, and a better parent, than earthly parents are; one who is everywhere present, and all-sufficient; that can't be kept off by infectious distempers; who is able to save from death or to make happy in death; to save from eternal misery and to bestow eternal life.It is indeed comfortable, when one is in great pain, languishing under sore sickness, to have the presence and kind care of near and dear earthly friends; but this is a very small thing, in comparison of what it is, to have the presence of a heavenly Father and a compassionate and almighty Redeemer. In God's favor is life, and his lovingkindness is better than life.
Whether you are in sickness or health, you infinitely need this.