Thomas Goodwin's teaching on assurance of salvation might be helpfully to analysed using John Frame's tri-perspectival approach to theology. For Goodwin, there are three aspects to assurance of salvation: 1) Faith in the promise of the gospel. 2) The testimony of grace- transformed life. 3) The direct witness of the Spirit.
The first point corresponds to John Frame's normative perspective. Saving faith places its trust in the authoritative promise of God found in Scripture, the 'norming norm'. The second point is in the realm of the situational perspective. With grace in his heart, the believer seeks to practice the gospel in every given situation. The third point brings in the existential perspective. It is about the believer's experience of the direct and personal witness of the Spirit, assuring him that he is a child of God.
Now, as Frame insists, while his three perspectives may be distinguished, they should not be divided. So with assurance, a profession of faith alone, apart from a changed life is no basis for assurance. Faith without works is dead. On the other hand, to focus on searching for grace in the heart, while loosing sight of the objective promise of salvation in Christ, is introspective and dangerous. We must also say that the Spirit will not give experiential assurance to those without genuine faith that works by love. All three perspectives must therefore be brought into play when seeking assurance of salvation, the normative - faith in Christ. The situational - the evidence of new life. And the existential - the witness of the Spirit. Yet, as assurance is about Christian experience, the most vital aspect is the direct witness of the Spirit, Romans 8:15-16.
Westminster Confession of Faith XVIII:II
"This certainty is not a bare conjectural and probable persuasion grounded upon a fallible hope;but an infallible assurance of faith founded upon the divine truth of the promises of salvation, the inward evidence of those graces unto which these promises are made,the testimony of the Spirit of adoption witnessing with our spirits that we are the children of God, which Spirit is the earnest of our inheritance, whereby we are sealed to the day of redemption."
For more on Goodwin and assurance see my Thomas Goodwin: His Life, Times, and Quest for Assurance, under Articles About Goodwin, here.