Richard Baxter, “The Reformed Pastor” Chapter 2, Section 2, #13, “With earnest desires and expectations of success.”

13. If you would prosper in your work, be sure to keep up earnest desires and expectations of success. If your hearts be not set on the end of your labors, and you long not to see the conversion and edification of your hearers, and do not study and preach in hope, you are not likely to see much success. As it is a sign of a false, self–seeking heart, that can be content to be still doing, and yet see no fruit of his labor; so I have observed that God seldom blesses any man’s work so much as his, whose heart is set upon the success of it. Let it be the property of a Judas to have more regard to the bag than to his work, and not to care much for what they pretend to care; and to think, if they have their salaries, and the love and commendations of their people, they have enough to satisfy them: but, let all who preach for Christ and men’s salvation, be unsatisfied until they have the thing they preach for. He never had the right ends of a preacher, who is indifferent whether he obtain them, and is not grieved when he misses them, and rejoiced when he can see the desired issue. When a man does only study what to say, and how, with commendation, to spend the hour, and looks no more after it, unless it be to know what people think of his abilities, and thus holds on from year to year, I must needs think that this man does preach for himself, and not for Christ, even when he preaches Christ, how excellency whatever he may seem to do it. No wise or charitable physician is content to be always giving physic, and to see no amendment among his patients, but to have them all die upon his hands. Nor will any wise and honest schoolmaster be content to be still teaching, though his scholars profit not by his instructions, but both of them would rather be weary of the employment.
I know that a faithful minister may have comfort when he wants success; and “though Israel be not gathered, our reward is with the Lord (Isa. 49:5);” and our acceptance is not according to the fruit, but according to our labor: but then, he who longs not for the success of his labors can have none of this comfort, because he was not a faithful laborer. What I say is only for them that are set upon the end, and grieved if they miss it. Nor is this the full comfort that we must desire, but only such a part as may quiet us, though we miss the rest. What if God will accept a physician, though the patient die? He must, notwithstanding that, work in compassion, and long for a better issue, and be sorry if he miss it. For it is not merely our own reward that we labor for, but other men’s salvation. I confess, for my part, I marvel at some ancient reverend men, that have lived twenty, thirty, or forty years with an unprofitable people, among whom they have scarcely been able to discern any fruits of their labors, how they can, with so much patience, continue among them. Were it my case, though I dare not leave the vineyard, nor quit my calling, yet I should suspect that it was God’s will I should go somewhere else, and another come in my place that might be fitter for them; and I should not be easily satisfied to spend my days in such a manner.