At all times in the history of the church, baptism has been the rite of initiation into church membership. In the first decades, baptism was given immediately in response to a declaration of faith in Christ (eg Acts 2:41, 8:36, 16:33). The pattern was to believe and be baptised, with little or no instruction beforehand. It is reasonable to assume that baptism was normally immersion, to symbolise the death and burial to the old life and the resurrection to the new life in Christ (Rom 6:1-4).
The Didache gives some helpful information about the mode of baptism in the early centuries of the church (7:1-4). It stated to baptise people in the three-fold name for God - Father, Son and Holy Spirit (as Matt 28:19). Also, that the one being baptised and the one doing the baptising should fast for two days beforehand. It gave preference for immersion in cold running water, or to use warm or still water if that was not available. It also gave the option to pour water three times on the head, if insufficient water was available.
By the second century, baptisms were only conducted once a year at Easter, presumably to symbolise the resurrection to new life. It was normally preceded by a lengthy period of instruction. This would be for at least two years, during which time the candidate was excluded from the Eucharist. It was likely that the person was immersed in running water three times, in the names of each member of the Trinity.
The Tradition of Hippolytus indicates that baptism was followed by the laying on of hands by the bishop to pray for the reception of the Holy Spirit, as well as the anointing of oil to drive out evil spirits. This act later developed into the sacrament of confirmation, which became separated from baptism as the use of infant baptism became more widespread.
In the early years, baptism was understood to be a requirement following a declaration of faith in Christ, for sins to be forgiven and to enter into the Kingdom of God, but was probably not understood to have power in itself to forgive sins. In the mid-second century however, Justin wrote that the act of baptism itself was the means of rebirth and the washing from past sin.
The baptism of infants gradually became popular in the early centuries until it became the normal mode of baptism, particularly once the church became the state church. By the time of Tertullian, it had become so prevalent that he warned against it, suggesting that baptism was delayed until the age of maturity. The use of infant baptism grew as the general understanding of baptism moved away from being a personal declaration of faith in Christ, to an understanding where baptism itself washed away sins. Augustine taught that the act of baptism removed the original sin inherited from Adam. As a result, baptism was increasingly seen as mystical act, and the belief that children who died before being baptised would not be saved. The other effect was that it concentrated more power into the hands of the clergy, as had changes in the understanding of the nature of the Eucharist.
On the other extreme, some wished to delay baptism to late in life, as the belief grew that major sins committed after baptism could not be forgiven. The Emperor Constantine was such an example, who delayed his baptism until he was on his death-bed.