English Puritanism was a “divided house”, there were those who followed William Perkins, and those who followed Richard Sibbes and John Cotton. The issue of division was oriented primarily around the concept of “assurance” of salvation. Sibbes and Cotton held the locus of contention was resolved with an “assurance of faith” (or simply knowledge of God and the objective criteria of God’s Word—illuminated in the heart of the believer); versus Perkins (and camp) who believed that sanctification and the practical syllogism, or external works, served as the final basis for determining if indeed a person was elect or not. John Cotton believed that John Calvin was in the former camp, and that “good works” or the “practical syllogism” were not the basis for determining one’s election, note:

And seeing we all profess . . . to hold forth protestant doctrine, let us hold it forth in the language of Calvin and others [of] our best protestants, who speak of purity of life and growth in grace and all the works of sanctification as the effects and consequents of our assurance of faith . . . . And therefore if we will speak as protestants, we must not speak of good works as cause or ways of our first assurance. . . . [Y]et indeed you carry it otherwise. . . . Which, seeing it disallowed by the chief protestant writers, if you contrary to them do hold it forth for protestant doctrine, that we may gather our first assurance of justification from our sanctification, it is not the change of words that will change that matter. (Ron Frost, unpublished PhD Dissertation, Richard Sibbes’ Theology of Grace and the Division of English Reformed Theology, King’s College, University of London 1996, 13, quoting Hall, Antinomian, 133-34, quoting John Cotton’s, Rejoinder)

Here Cotton is responding to the charge that he is an antinomian.

What I want to highlight is that historically “Calvinism” has more nuance to it than popularly understood today. There were “Free Grace Calvinists” (e.g. Richard Sibbes, John Cotton, et al), and there were “Federalist Calvinists” [”Law-Keeping”] (e.g. William Perkins, William Aames, et al). I appreciate the former, and believe with both Cotton and Calvin, that assurance of salvation is solely and objectively based upon the witness of GOD’S WORD, emphasizing God’s faithfulness in salvation, rather than my own. Like Cotton, I see justification as distinct, yet inseparably related to sanctification, and consequently hold that sanctification does not serve as the BASIS for anyone’s assurance of salvation . . . but that sanctification, in scripture, is the instrumental means through which the light of Christ is brought to bear on exposing the darkness of this world system. The consequence of sanctification, primarily, is to cause unbelieving man to see the Christian’s good works and glorify and praise God.