Historically when people became Christians they were baptised. This was originally
a ceremony in which adults were submerged in water as a symbol of their dying to
an old way of life and being born to a new. The church believes that the instruction
to be baptised came from Jesus:

 Therefore go and make disciples of all nations, baptising them in the
 name of the 
Father and of the Son and of the Holy Spirit, and teaching
 them to obey everything I have commanded you. And surely I am with
 you always, to the very end of the age.                                                                                                                                                                                                                                                                                      

Later the baptismal ceremony evolved into one whereby infants were welcomed into the
church. This is called infant baptism but sometimes known as christening. This is the
ceremony the Church of Scotland follows today. When children are older they can join the
church officially in a Service of Confirmation. If an adult has not been baptised as an infant
they can be baptised as an adult and then confirmed.