Baptism with water is, and has been for a long time (at least the 3rd Century AD (Burnish, Raymond, The Meaning of Baptism, (London:SPCK), 1985 p1). However, the centrality of baptism does not necessarily make one a Christian. While for the Anglican churches of England and Wales, baptism is the de-facto sign of membership, membership of a Church, nor baptism, makes one necessarily a Christian.
There is much writing on what makes one a Christian, the Marks of Christianity. Paul Tillich suggests that theological language and religious ritual as symbolic, and in and of themselves lack eternal truth, but they nevertheless point to the eternal and ultimate. He suggests that it is not ritual that is of importance, here, but rather Love. The love of God, and ultimately, God’s love for man (Tillich, Paul,Ultimate Concern, (London: SCM Press), 1965, p3). It is to this inner life change that Thomas Kempis points in his Imitations of Christ, where he asks, most earnestly, for his soul to be prepared for Christ, and to deny entrance to all others (Kempis, Thomas, The Imitation of Christ, (London: Fontana Books), 1963). It is interesting to note that we don’t have any record for the original apostles’ Baptism (not including Paul).
There is also much talk of a “Baptism of the Spirit”. This is something that is seen at Pentacost in the book of Acts(NRSV Acts 2). It is this baptism that is promised by Christ, and for some, this is the only true baptism that matters, to be born again from above. Some see that Baptism with the spirit (and in this case, the glossolalia (speaking in tongues) which accompany it) as being the mark of that confirms a new member in their faith (McGuire, Meredith B, Testimony as a Commitment Mechanism in Catholic Pentecostal Prayer Groups, (Wiley:Journal for the Scientific Study of Religion),1977, p 165 (link:http://www.jstor.org/stable/1385747 ). For many, it is simply enough to be “Saved”, and this is all that is required to be considered a Christian (http://carm.org/what-baptism-holy-spirit). This distinction, explored by John Wesley, suggests that it is only through an experience of God does one recieve the gift of entire sanctification, or as he would have it, Christian Perfection(Dunn, James D. G, Spirit Baptism and PEntecostalism (Scottish Journal Of Theology, Vol 23, issue 04), 1970, p 398). This leads onto the modern-day Holiness Movement, with it’s ‘Higher Life’ teaching in which justification by faith was separated from the second divine work of sanctification, which was also received by faith. No ritual was required, simply a personal commitment and faith in Jesus Christ as Lord and Saviour.
This notion is backed up by the bible, Math 7:16 suggests that the way to tell a true Christian from a fake one (in this case, prophet), is by their fruits, that is their actions.
It seems, therefore, that it is possible to be a Christian without baptism, or any other form of ritual.