I'm under the impression that the Christian god uses many methods to communicate with its devotees. Why not include literature amongst those methods?
From a historical context, it's worth remembering that Pagan religions were for the most part oral traditions. Putting things down in a text enabled things to be codified in ways that were somewhat foreign to humanity's religious modus operandi previously. It's my understanding that one of the big deals of Judaism was the creation and enforcement of law - codes laid down and intended to be unchanging. Christianity developed out of that ideal to some extent. But there are others who are more well-versed in the development of Abrahamic religions than I, so I defer to their expertise in full. 
Catholic Christianity, Judaism and Islam each have sacred
oral traditions (known as Sacred Tradition, Oral Torah and Sunnah respectively) in addition to our written scriptures (the New Testament, the Tanakh and the Qur'an).
More accurately, it could be said that Christianity developed out of
opposition to the theocratic legalism of the ancient Israelite faith, rather than in accordance with that ideal. As Pope Emeritus Benedict XVI explained in one of his speeches:
Apostolic Journey to Germany: Visit to the Federal Parliament in the Reichstag Building (Berlin, 22 September 2011) | BENEDICT XVI
"Unlike other great religions, Christianity has never proposed a revealed law to the State and to society, that is to say a juridical order derived from revelation. Instead, it has pointed to nature and reason as the true sources of law"
Legalism became synonymous with the heresy of "
Pelagianism" early on in Christian thought - the idea that salvation consists in vigorously following the requirements of an external religious law, the "
works of the law" as with the Mosaic Covenant. Instead, the Church Fathers taught salvation came from "
openness to grace" and the natural law accessible to our conscience, an interior law within our minds and not something externally imposed on us with prohibitions and commands as in: "
do this, do that".
The New Law of Christ does not consist of any "rules" in that way. It is spontaneous, interior, conscientious. It is about people searching deep within themselves for the voice of God, in openness to his saving grace and heeding the dictates of a well-formed conscience. This is why we have such things as the Sacrament of Confession and the practice of Examination of Conscience, used for instance by the Jesuits. There is no Christian Torah or Shariah law, nor even a detailed set of precepts as with the Buddha's Noble Eightfold Path.
The New Law is the inner grace of the Holy Spirit, accessible only to the individual's conscience. See this article written in 1962 by Stanislaus Lyonnet, a Jesuit priest and biblical scholar:
ST. PAUL: LIBERTY AND LAW – Women Can Be Priests
The law of the Spirit is radically different by its very nature. It is not just a code, not even one "given by the Holy Spirit", but a law "produced in us by the Holy Spirit"; not a simple norm of actions outside us, but somethings, that no legal code as such can possibly be: a new, inner, source of spiritual energy.
If St. Paul applies the term "law" to this spiritual energy, rather than the term "grace" that he uses elsewhere (see Rom.6:14) he most probably does it because of Jeremiah's prophecy (also mentioned in this context by St. Thomas) announcing a new covenant, the "New Testament" . For the prophet, too, speaks of law: "This is the covenant which I will make with the house of Israel . .. . I will place my law within their hearts" (31:33). Every time the Angelic Doctor refers to this "New Testament", he does so in the same terms: "It is God's way to act in the interior of the soul, and it was thus that the New Testament was given, since it consists in the inpouring of the Holy Spirit". Again: "It is the Holy Spirit Himself who is the New Testament, inasmuch as He works in us the love that is the fulness of the Law (23). For the Church and for her liturgy too, the promulgation of the New Law does not date from the Sermon on the Mount, but from the day of Pentecost when the "finger of the Father's right hand",digitus paternae dexterae, wrote His law in the hearts of men; the code of the Old Law, given on Sinai, finds its counterpart, not in a new code, but in the giving of the Holy Spirit."
From this fundamental doctrine everything else flows, notably,the fact that Christian morality is of necessity founded on love, as St. Paul following his Master, teaches:"The whole Law is fulfilled in one word: Thou shalt love thy neighbor as thyself" (Gal 5:14) "He who loves his neighbour has fulfilled the Law If there is any other command it is summed up in this saying: Thou shalt love thy neighbour as thyself .... therefore is the fulfilment of the Law" (Rom 13:8-10) The reason is that love is not first of all a norm of conduct, by a dynamic force. As St. Thomas notes, it is precisely because the Law, as a law, was not love that it could not justify man: "Consequently it was necessary to give us a law of the Spirit, who by producing love within could give us life." (27) .
Under these conditions, it is easy to see that a Christian, that is one led by the Holy Spirit, (28) can at the same time be freed from every external law - "not be under the law" - and yet lead a perfect moral and virtuous life. St Paul makes it abundantly clear in the epistle to the Galatians, shortly after he has reduced te whole law to love: "Walk in the Spirit, and you will not fulfil the lusts of the flesh (Gal. 5: 16) Nothing could be more obvious, he explains, since these are two antagonistic principles: If you follow me, you cannot but oppose the other." If you are led by the Spirit, you are not under the Law." In fact, what need would you have of law? A Spiritual man knows perfectly well what is carnal, and, if he is spiritual, he will fly from it as by instinct
It is about people being moral agents, responsibly forming and examining their own consciences over time - "working out their own salvation" - through the grace of God and the help of Holy Mother Church with her teachings, scriptures and sacraments, which exist to enlighten or "form" individual conscience properly but not to replace it.
