The common Chritian Faith -
(1) For the simple question "Why does a Christian need to go to Church?" we have available a simple answer: "Because Jesus told us to go", meaning this "remeber me in the bread and wine". I think to be a Christian is to follow Jesus. And this is to practicize His love for all, specially for the poorest and most excluded. Religion, Church and Mass are a kind of signs - sacraments - of this love and relantionship. Christians believe and love Jesus but Jesus today exist in the Christ Church. Not as an institution, but as a community of believers. Church is the Lord's body (St Paul) in human history; so, the Christ's body today is "we" too.
(2) Religion in general is a sacrament. For St. John's Gospel and traditional Faith in Trinity, God is Love. a kind of spiritual presence in all the creation (the Father), in all humanity (the incarnated Son) and in our spiritual life today (the Holy Spirit). In Liberation Theology it is used to say "The Hole Trinity is the best Community" - and the divine community should be our light for all efforts in shapping our societies in more egalitarian communions and participations. (L. Boff)
I really think this theological thinking is sound, but I know some christians ans specially some teologicians will think it not so good.
The common modern disbelief -
Many modern people would like to compatibilize the Christian Faith and the philosophical
and scientific modern culture. Modernity is no more religious, at least in the old or traditional sense.
One way for compatibilization is to deny modernity. Other is to deny religion. Other yet is to look for a synthese between the two. May be the third is the best or the truth, but I like the second, to deny religion.
It could be that religion was simply a part of the old or traditional world, a part of that culture, and culture is a secondary thing, like the patriarcal customms. Jesus was religious because his world were so, but this is not the point for christianity, that could be a way of life without religion, or at least, traditional religion. For example, the two topics above could be so reinterpreted:
(1) could be (1'): to be a Christian today would be a kind of humanist, like Jesus ("the saturday law was made for human beings, not human beings for the saturday law");
(2) could be (2'): to belive in God could be only to admire the universe, the humanity, and our spiritual practices. This could be a special kind of pantheism (see Spinoza's writings), or something like (an atheism/agnosticism? I do not know a theology that defends strongly the atheism/agnosticism vision, but I am sure that it exists! It might be Bonhoeffer's religionless Christiany!*).
What do you think?
*Father Martin Flatman (Oxford Brooks University) suggested me this. His other comments help so much and I thank him.