Seeking a fuller definition of conscience

Cardinal Newman defines conscience as the "aboriginal vicar of Christ," according to the triple munera which are rooted in the Old Testament figures of priest, prophet and king, which Christ assimilates unto Himself and perfects, and which the Church is given as gifts and duties. 


Conscience is the aboriginal Vicar of Christ, a prophet in its informations, a monarch in its peremptoriness, a priest in its {249} blessings and anathemas, and, even though the eternal priesthood throughout the Church could cease to be, in it the sacerdotal principle would remain and would have a sway.


http://www.newmanreader.org/works/anglicans/volume2/gladstone/section5.html


The conscience, in other words, exists in a manner more fundamental that any other manifestation of God in our human experience.  Even without any other religious experience, man still possesses the voice of God deep in his heart.  Conscience proclaims the truth like a prophet, rules over our actions expecting obedience as a king, and blesses or condemns the choices we face as a priest blesses or condemns heresy. 


Therefore, conscience cannot be the opinion of the individual but, rather, the voice of God Himself in the heart of His creature - the pinnacle of His creation, made in his image, capable of union with Him and able to love Him in return - the human person. 


Gaudium et spes no. 16 concurs:


In the depths of his conscience, man detects a law which he does not impose upon himself, but which holds him to obedience. Always summoning him to love good and avoid evil, the voice of conscience when necessary speaks to his heart: do this, shun that. For man has in his heart a law written by God; to obey it is the very dignity of man; according to it he will be judged.(9) Conscience is the most secret core and sanctuary of a man. There he is alone with God, Whose voice echoes in his depths.(10) In a wonderful manner conscience reveals that law which is fulfilled by love of God and neighbor.(11) In fidelity to conscience, Christians are joined with the rest of men in the search for truth, and for the genuine solution to the numerous problems which arise in the life of individuals from social relationships. Hence the more right conscience holds sway, the more persons and groups turn aside from blind choice and strive to be guided by the objective norms of morality. Conscience frequently errs from invincible ignorance without losing its dignity. The same cannot be said for a man who cares but little for truth and goodness, or for a conscience which by degrees grows practically sightless as a result of habitual sin.