Some of his books were totally devoted to the Puritans, including his 1990 volume, A Quest for Godliness: The Puritan Vision of the Christian Life (Crossway). I have already discussed this book in various articles. Six years ago I penned a piece on his chapter on Puritan preaching: https://billmuehlenberg.com/2018/08/29/packer-preaching-and-the-puritans/

Here I want to quote from his seventh chapter which discusses "The Puritan Conscience." In it he focusses on three major topics of interest to the Puritans and how they relate to conscience:

First, this teaching reflects the Puritan view of Holy Scripture. God, said the Puritans, must control our consciences absolutely....

The Puritans themselves sought clear certainty as to God's truth in its practical bearing, and believed that they had been given it. Their very quest sharpened both their moral sensibilities and their insight into the Bible. They would not have been interested in vague moral uplift; what they wanted was to grasp God's truth with the same preciseness of application with which they held that He had revealed it. Because of their concern for preciseness in following out God's revealed will in matters moral and ecclesiastical, the first Puritans were dubbed 'precisians.' Though ill-meant and derisive, this was in fact a good name for them. Then as now, people explained their attitude as due to peevish cantankerousness and angularity or morbidity of temperament, but that was not how they themselves saw it....

Second, the Puritans' teaching on conscience reflected their view of personal religion. Godliness, to the Puritans, was essentially a matter of conscience, inasmuch as it consisted in a hearty, disciplined, 'considerate' (thoughtful) response to known evangelical truth, and centred upon the getting and keeping of a good conscience. As long as a man is unregenerate, his conscience oscillates between being bad and being asleep. The first work of grace is to quicken his conscience and make it thoroughly bad, by forcing him to face God's demands upon him and so making him aware of his guilt, impotence, rebelliousness, defilement, and alienation, in God's sight. But the knowledge of pardon and peace through Christ makes his bad conscience good. A good conscience is God's gift to those whom, like Bunyan's pilgrim, he enables to look with understanding at the cross. It is maintained through life by seeking to do God's will in all things, and by constantly keeping the cross in view....

A good conscience, said the Puritans, is the greatest blessing that there is. 'Conscience,' declared Sibbes, 'is either the greatest friend or the greatest enemy in the world.' There is no better friend than a conscience which knows peace with God. (pp. 112-115)

Packer continues:
A good conscience is a tender conscience. The consciences of the godless may be so calloused that they scarcely ever act at all; but the healthy Christian conscience (said the Puritans) is constantly in operation, listening for God's voice in his word, seeking to discern his will in everything, active in self-watch and self-judgement. The healthy Christian knows his frailty and always suspects and distrusts himself, lest sin and Satan should be ensnaring him unawares; therefore he regularly grills himself before God, scrutinising his deeds and motives and ruthlessly condemning himself when he finds within himself moral deficiency and dishonesty. This was the kind of self-judging that Paul urged upon the Corinthians at Communion time (1 Cor 11:31). The degree of sharp-sightedness which our consciences show in detecting our own real sins (as distinct from the imaginary ones on which Satan encourages us to concentrate) is an index of how well we really know God and how close to him we really walk--an index, in other words, of the real quality of our spiritual life. The sluggish conscience of a 'sleepy', 'drowsy' saint is a sign of spiritual malaise. The healthy Christian is not necessarily the extrovert, ebullient Christian, but the Christian who has a sense of God's presence stamped deep on his soul, who trembles at God's word, who lets it dwell in him richly by constant meditation upon it, and who tests and reforms his life daily in response to it. We can begin to assess our real state in God's sight by asking ourselves how much exercise of conscience along these lines goes into our own daily living.

Third, the Puritans' teaching on conscience was reflected in their view of preaching. The most characteristic feature in the Puritan ideal of preaching was the great stress laid on the need for searching applications of truth to the hearers' consciences. One mark of a 'spiritual', 'powerful' preacher, in the Puritan estimation, was the closeness and faithfulness of application whereby he would 'rip up' men's consciences and make them face themselves as God saw them. The Puritans knew that sinful men are slow to apply truth to themselves, quick though they may be to see how it bears on others. Hence unapplied general statements of evangelical truth were unlikely to do much good. Therefore (said the Puritans) the preacher must see it as an essential part of his job to work out applications in detail, leading the minds of his hearers step by step down those avenues of practical syllogisms which will bring the word right home to their hearts, to do its judging, wounding, healing, comforting, and guiding work. (pp. 117-117)

Two major objections to the Puritans in all this are raised by Packer, and he answers both quite carefully and skillfully:

It may be asked: does not this stress on the searching of conscience produce a morbid and introspective type of piety? Does not this emphasis on constant self-suspicion and self-examination actually weaken faith, by diverting our gaze from Christ in his fulness to ourselves in our emptiness, so leading us to spiritual despondency and depression? No doubt it would if it were made an end in itself; but, of course, it never was. The Puritans ripped up consciences in the pulpit and urged self-trial in the closet only in order to drive sinners to Christ and to teach them to live by faith in him. They plied the law only to make way for the gospel, and for the life of dependence on the grace of God. Morbidity and introspectiveness, the gloomy self-absorption of the man who can never look away from himself, is bad Puritanism; the Puritans themselves condemned it repeatedly. A study of Puritan sermons will show that the preachers' constant concern, in all their detailed detecting of sins, was to lead their hearers into the life of faith and a good conscience; which, they said, is the most joyous life that man can know in this world.

The Puritan concern for a good conscience lent great ethical strength to their teaching. Of all English evangelicals from the Reformation to the present day, the Puritans were undoubtedly the most conspicuous as preachers of righteousness. They were in truth the salt of society in their time, and on many points they created a national conscience which has only recently begun to be eroded. A demand for the sanctification of the Sabbath; plain speaking against demoralising amusements (bawdy plays, promiscuous dancing, gluttony and drunkenness, salacious fiction); abhorrence of profanity; insistence on a faithful management of one's calling and station in life--these were emphases which are still remembered (sometimes applauded, sometime ridiculed) as 'Puritan'. Just as Laud had a policy of 'thorough' in ecclesiastical affairs, so the Puritans had a policy of 'thorough' in ethical realm; and they went to great pains to give detailed guidance on the duties involved in the various relationships to God and man in which the Christian stood. Among the memorials of their work in this field are the many printed expositions of the Ten Commandments; major works like Richard Rogers' Seven Treatises ... the Practice of Christianity (1603), Perkins' and Ames' volumes on conscience and casuistry, and Baxter's Christian Directory (1670); plus countless small vade-mecums on the Christian life, from Arthur Dent's Plain Man's Pathway to Heaven (1601) down to Thomas Gouge's Christian Directions Shewing how to Walk with God All the day Long (1688).

Was all this detailed teaching on Christian conduct a lapse into a new legalism and a curtailing of Christian liberty? Does it mark a decline into pharisaic ways?

No; for first, all this ethical teaching was evangelically based, as that of the New Testament is. The supreme ethical motives in Puritanism were gratitude for grace received, and a sense of responsibility to walk worthy of one's calling, and there was not the least room in Puritan teaching for self-righteousness; for not only was it constantly stressed that the Christian works from life, rather than for life, but it was also repeatedly emphasised that our best works are shot through with sin, and contain something that needs to be forgiven.

Then, secondly, this ethical teaching was all given (again, just as in the New Testament) not as a code of routine motions to go through with mechanical exactness, but in the form of attitudes to be maintained and principles to be applied, so that however much teaching and advice a man received, he was always left to make the final decisions and determinations (whether to follow his pastor's advice; how to apply the given principles in this or that case; etc) on his own initiative, as spontaneous, responsible acts of his own conscience in the sight of God.

Thirdly, Puritan ethical teaching was not authoritarian; it was offered as exposition and application of Scripture, and was to be checked against Scripture by those who received it, according to the Protestant principle of the duty of private judgement. The Puritans did not wish men's consciences to be bound to their own teaching, as such, but to the word of God only, and to Puritan teaching only so far as it was demonstrably in accord with that word.

Fourthly, Puritan ethical teaching took the form of a positive ideal of zealous and wise godliness, at which Christians must always be aiming even though they never fully reach it in this world; and unattained positive ideals are the death of the legalistic spirit, which can only flourish in an atmosphere of negative restriction where abstinence is regarded as the essence of virtue. In reality nothing less legalistic in spirit and content than the ethical teaching of the Puritans can well be imagined. (pp. 117-119)

He closes the chapter this way:

So the conclusion I would draw is simply this: that such conscientiousness as marked all Puritan religion, and was supremely manifested in the ejections of 1662, is a necessary Christian virtue at all times. It is man's proper response to God's immutable revealed truth. It may be costly, as it was in 1662; but without it, churchmanship becomes irreligion, and one's Christian profession becomes an insult to God. These are compromising days in the church's life; that, perhaps, is only to be expected when the very existence of revealed truth is so widely doubted or denied. But if we believe that God has spoken in his Son, and that the Bible is his own word of testimony to that revelation--if, in other words, we hold the Puritan view of Scripture--then, as we said earlier, the uncompromising fidelity to Bible truth which marked the Puritans should mark us also. May God give us light to see his truth, consciences to apply it and live by it, and conscientiousness to hold it fast, whatever the cost, in these Laodicean days. (pp. 121-122)

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