I recently read Christopher Ash's 'Pure Joy - Rediscover Your Conscience'. It is very helpful and inspired me to read Ole Hallesby's little book, entitled simply 'Conscience'. I have had it on my shelf for years but have not read it before, as far as I remember.
Hallesby is not exactly a Puritan but comes out in the end with a remarkably orthodox doctrine of conscience and its place in conversion and the Christian life. It is a stimulating book full of thought-provoking insights.
In one place he defines conscience as 'the consciousness of self in relation to God - the vital link between man's self-consciousness and his God-consciousness'. Conscience, he says, pronounces judgement according to the knowledge of God's will which a person possesses at the time. It is a universal faculty and exercises the same function (judgement as to moral acts, thoughts words and omissions - this is the 'form' of conscience) in an unbeliever as in a believer. It is deficient however in its content (though the 'form'/ 'function' of conscience is the same) insofar as a person's knowledge of the law of God is deficient. This is the great damage done to conscience by the Fall - its relatively bad, though still existent, knowledge of God's law; though in 'form' it is deficient too in that the voice of conscience is weak as to clarity and its 'volume' is less. But basically it is still the voice of God in man - not infallible , like God's law, but God's fallen vice-regent in fallen man, still to be heeded because it is the closest to God's true voice in us, and we ignore it at our peril.
Hallesby insists that the conscience must be awakened by regeneration to be of use to us in salvation. Once awakened it is the faculty through which the law 'kills' the sinner (Rom 7:9,10' Galatians 2:19). Hallesby is challenging in his criticism of evangelical preachers who do not press home the law hard enough - they go a little way, but too quickly let the sinner off the hook, because they are too afraid of this 'killing' and want the sinner to go straight to grace.
Hallesby says that typically a sinner once convicted will move (i) from trusting his own deeds to get him right with God (to make God love him), then, once seeing the futility of this, to (ii) trusting to what Christ does in him, then finally when all fails (conscience being the operative organ in making him distrust himself, as his knowledge of God's law increases) (iii) he will see that it is only what Christ has done for him, outside of him, that gives him assurance of salvation.
The believer, far from no longer needing conscience, finds his conscience more and more active, which is why maturer believers find more of sin in themselves, as the regenerate heart is increasingly aware of God's law. The cry of Joseph 'How can I ...sin against God' is the spiritually minded person's attitude to sin - hating it not because of what it does to me, but because of what it is in God's sight. The person growing in grace comes to love conscience, and love God's law, as his love for God grows. The believer will love to hear preaching on the law as well as the gospel (O! how some people need to hear that today!). We obey the law because he loves us, not so that he may love us.
There are two equal and opposite dangers: (i) to fall into thinking that it is our keeping of the law that keeps God loving us and (ii) to lessen the demands of the law so we can do it, so that the tension between the law's demand and the law's impossibility is relaxed and the whole dynamic of grace is lost.
Spend time in prayer, counsels Hallesby, for in prayer we allow conscience time to speak to us. A sensitive conscience is the key to spiritual health, and this is not in a sacred sphere as opposed to a secular, because a sensitive conscience makes us live all of life 'coram Deo' (before God).