I'm doing some reading on the development of the rite of Confirmation, especially as it relates to catechesis. There's quite the confusion today about what Confirmation is, or what it ought to be, especially in the Anglican tradition. It's evolved over the years and in some cases the theological rationale behind it is not always clear. But in general, for those traditions that baptize infants, Confirmation will be the main sacrament (or sacramental rite) that catechesis prepares one for.
The following passage is from a Pentecost homily attributed to the fifth-century Italian bishop, Faustus of Riez. Faustus was a defender of Nicene theology (interesting that people were still battling "Arianism" in the fifth century!), and of a more semi-Pelagian bent when it came to the doctrine of grace (he disagreed with Pelagius but also distanced himself from a strong Augustinian predestinarian view).
This is one of the earliest references we have to the rite of confirmation being logically distinguished from the rite of baptism, with the "laying on of hands" by the bishop as its defining gesture. In earlier periods, a rite similar to Confirmation occurred immediately after Baptism, and before one received Communion for the first time. As infant baptism slowly became the norm (largely due to the influence of Augustine), Confirmation came to be more its own rite, while still remaining the pre-requisite for receiving the Eucharist.
What I am particularly interested in here is how Faustus distinguishes between Baptism and Confirmation: Baptism gives life, while Confirmation strengthens life, though both are the work of the same Spirit.
"What the imposition of the hand bestows in confirming individual neophytes [the newly baptized], the descent of the Holy Spirit gave people then in the world of believers.… the Holy Spirit, who descends upon the waters of baptism by a salvific falling, bestows on the [baptismal] font a fullness toward innocence, and presents in confirmation an increase for grace.
And because in this world we who will be prevailing must walk in every age between invisible enemies and dangers, we are reborn in baptism for life, and we are confirmed after baptism for the strife. In baptism we are washed; after baptism we are strengthened.
And although the benefits of rebirth suffice immediately for those about to die, nevertheless the helps of confirmation are necessary for those who will prevail. Rebirth in itself immediately saves those needing to be received in the peace of the blessed age. Confirmation arms and supplies those needing to be preserved for the struggles and battles of this world. But the one who arrives at death after baptism, unstained with acquired innocence, is confirmed by death because one can no longer sin after death." (Quoted from Maxwell Johnson, Rites of Christian Initiation, rev. ed., 185)
There's a lot packed in this section, and it warrants a slow reading. But the main assumption, very prevalent in the early church, is that life in this world is a struggle, and we need the power of the Holy Spirit to strengthen (to "make firm") us in this present age against the wiles of the devil. Confirmation is spoken of here as "increasing" the grace that one receives in Baptism. Where in Baptism, the Holy Spirit bestows a "fullness towards innocence," in Confirmation the same Spirit gives "an increase for grace."
There's also a certain "this-worldly otherworldliness" to Confirmation. Faustus assumes that the goal of the Christian life is not for a person to get baptized, die the next day, and then go to heaven (although if that happens apart from Confirmation that's okay). Rather, it is assumed that the newly baptized Christian is going to need strengthening through the power of the Spirit for the "invisible enemies and dangers" that he or she will face in earthly life. The struggle is real, and it takes place in time and space. But it is also a spiritual battle, which occurs in and with those things we cannot see.
For all this we need to the "increased grace" given to us in the sacrament of Confirmation. Being born, we now live. And in living, we live not through our own powers but by the power and grace of the Holy Spirit.