The success of your labours materially depends on your taking heed to yourselves. pg 80

(3) Do you think it is a likely thing, that he will fight against Satan with all his might, who is himself a servant to Satan? Will he do any great harm to the kingdom of the devil, who is himself a member and a subject of that kingdom? Will he be true to Christ who is in covenant with his enemy? Now, this is the case of all unsanctified men, of whatever rank or profession they be. They are the servants of Satan, and the subjects of his kingdom; it is he that rules in their hearts; and are they like to be true to Christ that are ruled by the devil? What prince will choose the friends and servants of his enemy to lead his armies in war against him?
This is it that has made so many preachers of the gospel to be enemies to the work of the gospel which they preach. No wonder if such deride the holy obedience of the faithful; and if while they take on them to preach a holy life, they cast reproaches, on them that practice it! O how many such traitors have been in the Church of Christ in all ages, who have done more against him, under his colors, than they could have done in the open field! They speak well of Christ and of godliness in the general, and yet slyly do what they can to bring them into disgrace, and make men believe that those who set themselves to seek God with all their hearts are a company of enthusiasts or hypocrites. And when they cannot for shame speak that way in the pulpit, they will do it in private among their acquaintance. Alas! how many such wolves have been set over the sheep! If there was a traitor among the twelve in Christ’s family, no wonder if there be many now. It cannot be expected that a slave of Satan, “whose god is his belly, and who minds earthly things,” should be any better than “an enemy to the cross of Christ (Php_3:18).” What though he live civilly, and preach plausibly, and maintain outwardly a profession of religion? He may be as fast in the devil’s snares, by worldliness, pride, a secret distaste of diligent godliness, or by an unsound heart that is not rooted in the faith, nor unreservedly devoted to Christ, as others are by drunkenness, uncleanness, and similar disgraceful sins. Publicans and harlots do sooner enter heaven than Pharisees, because they are sooner convinced of their sin and misery (Mat_21:31).
And, though many of these men may seem excellent preachers, and may cry down sin as loudly as others, yet it is all but an affected fervency, and too commonly but a mere useless bawling; for he who cherishes sin in his own heart does never fall upon it in good earnest in others. I know, indeed, that a wicked man may be more willing of the reformation of others than of his own, and hence may show a kind of earnestness in dissuading them from their evil ways; because he can preach against sin at an easier rate than he can forsake it, and another man’s reformation may consist with his own enjoyment of his lusts. And, therefore, many a wicked minister or parent may be earnest with their people or children to amend, because they lose not their own sinful profits or pleasures by another’s reformation, nor does it call them to that self–denial which their own does. But yet for all this, there is none of that zeal, resolution, and diligence, which are found in all that are true to Christ. They set not against sin as the enemy of Christ, and as that which endangers their people’s souls. A traitorous commander, that shoots nothing against the enemy but powder, may cause his guns to make as great a sound or report as those that are loaded with bullets; but he does no hurt to the enemy. So one of these men may speak as loudly, and mouth it with an affected fervency, but he seldom does any great execution against sin and Satan.
No man can fight well, but where he hates, or is very angry; much less against them whom he loves, and loves above all. Every unrenewed man is so far from hating sin to purpose, that it is his dearest treasure. Hence you may see, that an unsanctified man, who loves the enemy, is very unfit to be a leader in Christ’s army; and to draw others to renounce the world and the flesh, seeing he cleaves to them himself as his chief good.