As leader of the Scottish Reformation, John Knox (1513-1572) lived in a time of religious turmoil, when false teaches were plentiful and lay believers were often unsure as to where firm foundations could be found. Thus in his Letter to the Scottish Brethren, Knox beseeches his audience to be on their guard against those sent by Satan to twist the truth, but also extends his confidence in the work of the Spirit within the church. The Spirit, he argues, will comfort the simple and mitigate the damage caused by those bringing selfish motives and evil doctrine.
For, as I have often told you before and now tell you again even with tears, many live as enemies of the cross of Christ.
Commentary on Philippians 3:18
For as the matter and business you have in hand is high, and to the advancement of God’s glory, and to no small comfort and consolation of your brethren; if in the same constancy with godly wisdom you proceed, so shall Satan be the most vigilant to trouble and impede the same, by all means possible; the powers of the earth shall no doubt stand against you; and the dissolute life and ungodly behavior, perchance, even of some preachers may slander and offend weak ones in Christ. But neither of both (except, as God forbid, you turn back from your godly enterprise) do I so much fear as the assault of Satan by false teachers or dissembling brethren; for seldom it is that open tyranny does utterly suppress (in any realm or province) the true religion earnestly received by a multitude. And albeit the ungodly life of preachers for a time troubles the quietness of some consciences, yet such is the mercy of our God toward his own elect that by his Holy Spirit he comforts the simple; and by the power of his blessed Word, in the end, he confounds the dissembling preacher or professor (I mean him that is the mercenary, who seeks the belly, and not the glory of Christ Jesus), so that neither the open tyranny, neither yet the cloaked and disguised preacher, can hurt much. But deceitful and false doctrine is a poison and venom which, under the taste and name of truth, once drunk and received, can only be purged with great difficulty later.
Philippians, Colossians, ed. Graham Tomlin, Reformation Commentary on Scripture, NT vol. XI, p. 91-92.