Most Christians today, at least in the developed world, are baptized in infancy; and even those whose traditions delay the rite until adulthood are, for the most part, children of Christian families and have grown up in the faith, and so their baptisms merely seal and affirm the lives they have always lived. This was obviously not the case, however, for most of the Christians of the earliest centuries; for them, baptism was of an altogether more radical nature. It was understood as nothing less than a total transformation of the person who submitted to it; and as a ritual event, it was certainly understood as being far more than a mere dramaturgical allegory of one’s choice of religious association. To become a Christian was to renounce a very great deal of what one had known and been to that point, in order to be joined to a new reality, the demands of which were absolute; it was to depart from one world, with an irrevocable finality, and to enter another.
A convert to Christianity from paganism somewhere in, say, the greater Byzantine world, within the first few decades after the Edict of Milan, would not in most circumstances have been granted immediate entry into the community of the faith.
Catechetical and liturgical customs varied greatly from place to place, but certain aspects of Christian baptism were very nearly universal. In general, if one sought to be received into the church, one had first to become a catechumen, a student of the church’s teachings….. one might typically be required to depart from the congregation on Sundays after the liturgy of the word, before the Eucharist was celebrated. And one could remain in this liminal state, in many cases, for years, receiving instruction, submitting to moral scrutiny, learning to discipline one’s will, and gradually becoming accustomed to the practice of the Christian life. Whether brief or protracted, however, the period of one’s preparation for baptism could not conclude until one had been taught the story of redemption: how once all men and women had laboured as slaves in the household of death….. and how Christ had come to set the prisoners free and had, by his death and resurrection, invaded the kingdom of our captor and overthrown it, vanquishing the power of sin and death in us…
For it was into this story that one’s own life was to be merged when one at last sank down into the “life-giving waters”: in the risen Christ, a new humanity had been created, free from the rule of death, into which one could be admitted by dying and rising again with Christ in baptism…..
-- David Bentley Hart in "Atheist Delusions"
It is a peculiarity of our modern age that many people who identify themselves as Christians actually know - and understand - very little about Christianity. They enter churches singing along and smiling amiably but the reality is that their grasp of even the most fundamental tenets of the faith is weak, even tenuous.
Part of the problem is this: in the rush to be inclusive and welcoming, many churches make the mistake allowing newcomers into the life of the church without first ascertaining their understanding of Christ and his redemptive work.
This typified by practices such as allowing non members into service and participation in nearly every part of the church's life and a very truncated period of teaching and preparation for baptism. In at least one church I attended, preparation for baptism consisted of a 2 hour session, half of which was taken up by an explanation of the logistics of the actual baptism service. There was no inquiry into the life of the candidate save for ascertaining that the candidate had attended the church regularly for at least 6 months.
It is unfortunate, but in most cases, a newcomer's willingness to be of service and friendliness are mistaken as conversion and transformation when it is possible (or even likely) that neither have taken place.