MISSION TO BELIEVERS
The Church does not feel dispensed from paving unflagging attention also to those who have received the faith and who have been in contact with the Gospel often for generations. Thus she seeks to deepen, consolidate, nourish and make ever more mature the faith of those who are already called the faithful or believers, in order that they may be so still more.
This faith is nearly always today exposed to secularism, even to militant atheism. It is a faith exposed to trials and threats, and even more, a faith besieged and actively opposed. It runs the risk of perishing from suffocation or starvation if it is not fed and sustained each day. To evangelize must therefore very often be to give this necessary food and sustenance to the faith of believers, especially through a catechesis full of Gospel vitality and in a language suited to people and circumstances.
The Church also has a lively solicitude for the Christians who are not in full communion with her. While preparing with them the unity willed by Christ, and precisely in order to realize unity in truth, she has the consciousness that she would be gravely lacking in her duty if she did not give witness before them of the fullness of the revelation whose deposit she guards.
The second sphere is that of those who do not practice. Today there is a very large number of baptized people who for the most part have not formally renounced their Baptism but who are entirely indifferent to it and not living in accordance with it. The phenomenon of the non practicing is a very ancient one in the history of Christianity; it is the result of a natural weakness, a profound inconsistency which we unfortunately bear deep within us. Today however it shows certain new characteristics. It is often the result of the uprooting typical of our time. It also springs from the fact that Christians live in close proximity with non-believers and constantly experience the effects of unbelief. Furthermore, the non-practicing Christians of today, more so than those of previous periods, seek to explain and justify their position in the name of an interior religion, of personal independence or authenticity.
Thus we have atheists and unbelievers on the one side and those who do not practice on the other, and both groups put up a considerable resistance to evangelization. The resistance of the former takes the form of a certain refusal and an inability to grasp the new order of things, the new meaning of the world, of life and of history; such is not possible if one does not start from a divine absolute. The resistance of the second group takes the form of inertia and the slightly hostile attitude of the person who feels that he is one of the homily, who claims to know it all and to have tried it all and who no longer believes it.
Atheistic secularism and the absence of religious practice are found among adults and among the young, among the leaders of society and among the ordinary people, at all levels of education, and in both the old Churches and the young ones. The Church's evangelizing action cannot ignore these two worlds, nor must it come to a standstill when faced with them; it must constantly seek the proper means and language for presenting, or representing, to them God's revelation and faith in Jesus Christ.
Culled from - http://w2.vatican.va/content/paul-vi/en/apost_exhortations/documents/hf_p-vi_exh_19751208_evangelii-nuntiandi.html