Confirmation began as part of a single, unified rite of Christian initiation performed together with baptism, water, anointing, and first participation in the Eucharist administered in one continuous ceremony, a pattern Eastern Orthodox practice has preserved essentially intact to this day, where infants are baptized, chrismated, and given communion together in a single service rather than the three being spread across a childhood and adolescence.
The Western church's separation of confirmation into its own later, distinct sacrament is generally traced to a practical constraint rather than a deliberate theological decision to split the rite: the anointing that completed initiation was reserved to the bishop, and as Christianity spread across increasingly large territories a bishop could no longer be present at every baptism the way earlier, smaller, city centered congregations had allowed, so the anointing was delayed until a bishop could visit and confirm those already baptized, a scheduling gap that over centuries hardened into confirmation's current status as a separate sacrament administered at a distinctly later age from baptism itself. This is why Eastern and Western Christianity, despite agreeing that baptism, chrismation or confirmation, and the eucharist together complete Christian initiation, present that same theological structure through very different lived timelines, one keeping all three together in infancy and the other spreading them across a person's early life as a matter of historical accident rather than differing theology.