To most Christians, the way one "becomes a Christian" is by baptism. The way one "remains" a Christian is by communion. This is the majority view of the matter.
I understand that it is not the only view. Some Christians believe you become a Christian by repentence and conversion, not by the ritual of baptism. Protestants of Anabaptist or Calvinist ancestry (that is, all the Protestant sects other than Episcopals and Lutherans, which remain close to Roman Catholic roots) reject ritual and ecclesiastical authority. In reaction to the authoritarian position of the Roman Catholic Church, such Christians typically ascribe to themselves an objective or scientific viewpoint, purporting to find the "truth" of the matter by examining "for themselves" the evidence in the Scriptures. Ritual, especially ritual which must be performed by an initiated priest of the official hierarchy, is anathema to such Christians.
Fundamentalists, in particular, frequently invoke Romans 10:9 as a method whereby one "becomes" a Christian. It requires neither priest nor church to confess Jesus as one's personal savior. Please note, however, that even fundamentalists do not mean that uttering the incantation "Jesus is Lord" has any magical properties. Confessing that "Jesus is Lord" is the outward manifestation of a inner conversion experience. Blaise Pascal, a Jansenist, for example, recorded his own conversion experience in rushed, incoherent phrases. He had the writeup sown into his coat, to keep it literally next to his heart. Clearly this was a powerful and significant event in his life, much more sincere, deep and passionate than the rhetorical arguments for which Pascal is remembered (i.e. "the Wager)".
Christians to whom a conversion experience is important, and synonymous with Christian spirituality, find it absurd to think that a ritual baptism performed on an infant can make you a Christian. The truth of the Gospel, these Christians believe, can only be apprehended by an adult, or at the very least, a child who has reached the age of reason. Christians who rejected infant baptism were labeled "anabaptists" (re-baptisers) because they favored an adult baptism after a conversion experience, as manifested or confirmed by a public expression (or "confession") of faith. Nowadays, no sects identify themselves as Anabaptists because "Anabaptism" became notorious for the radical political agendas and communism of certain 16th century German Anabaptists. Nonetheless, many Protestants are in fact "Anabaptist" in the sense that they reject infant baptism in favor of adult conversion.
I'm not one of them. I believe that I became a Christian when I was baptised as an infant less than a month after I was born. It doesn't matter that I didn't understand what was going on, and lacked the capacity to discuss the matter with my parents and my pastor. I am a traditional Lutheran, and so believe the truth of Christianity is apprehended by faith, not by reason. Credo ut intelligam: (I believe in order to understand). If the baby Jesus can be God, why can't a baby become a Christian? Faith and grace operate through the sacraments, whether or not we understand or agree with them.