Why is SINCERITY of upmost importance in ministry?

Please allow me to let Thomas Watson and Richard Baxter, two 17th century Puritan pastors, answer this question:

Thomas Watson (exerpt from The Godly Man's Picture, p. 97-102)
"The godly man (of sincerity) is what he seems to be... A hypocrite is not what he seems. A godly man answers to his profession as the transcript to the original... The hypcrite thinks of nothing but self-interest, the sails of his mill move only when the wind of promotion blows....The only way to be safe is to be sincere...Sincerity is what the devil attacks most. Satan's spite was not so much at Job's estate, as his integrity; he would have wrested the shield of sincerity from him, but Job held that fast (Job 27:6). The thief does not fight for an empty purse, but for money. The devil would have robbed Job of the jewel of a good conscience, and then he would have been poor Job indeed. Satan does not oppose profession, but sincerity. Let men go to church and make glorious pretences of holiness. Satan does not oppose this; this does him no hurt and them no good; but if men want to be sincerely pious, then Satan musters up all his forces against them. Now what the devil most assaults, we must strive most to maintain. Sincerity is our fort royal, where our chief treasure lies. This fort is most shot at, therefore let us be more careful to preserve it. While a man keeps his castle, his castle will keep him. While we keep sincerity, sincerity will keep us.

Sincerity is the beauty of a Christian. Wherein does the beauty of a diamond lie, but in this, that it is a true diamond? If it is counterfeit, it is worth nothing. So wherein does the beauty of a Christian lie, but in this, that he has truth in the inward parts (Ps. 51:6)?...The hypocrite is an impudent sinner. He knows his heart is false, yet he goes on. All the plagues and curses written in the Book of God are the hypocrite's portion, hell is his place of rendezvous (Matt. 24:51). Hypocrites are the chief guests the devil expects and he will make them as welcome as fire and brimstone can make them."

Richard Baxter (exerpt from The Reformed Pastor, p. 61-72)
"Take heed to yourselves, lest your example contradict your doctrine, and lest you lay such stumbling-blocks before the blind, as may be the occasion of their ruin; lest you unsay with your lives, what you say with your tongues; and be the greatest hinderers of the success of your own labors... He that means as he speaks, will surely do as he speaks. One proud, surly, lordly word, one needless contention, one covetous action, may cut the throat of many a sermon, and blast the fruit of all that you have been doing....It is a palpable error of some ministers, who make such a disproportion between their preaching and their living; who study hard to preach exactly, and study little or not at all to live exactly. All the week long is little enough, to study how to speak two hours; and yet one hour seems too much to study how to live all the week....A practical doctrine must be practically preached. We must study as hard how to live well, as how to preach well....Take heed to yourselves, lest you live in those sins which you preach against in others, and lest you be guilty of that which daily you condemn....O sirs, how many men have preached Christ, and yet have perished for want of a saving interest in him! How many, who are now in hell, have told their people of the torments of hell, and warned them to escape from it!"