Water is used to indicate a multiplicity of biblical events and concepts: the washing away of sin, the walk of the Israelites to freedom through the waters of the sea, Noah's salvation through the waters of the Flood, the baptism of the Lord; the water that flowed from Jesus's side on the Cross. But foremost, having water poured on is a reminder of the immersion into water that symbolizes death to one's sinful self and rebirth to new life.
Oil is used to denote the Spirit, who strengthens and protect, much as oil did strengthen and protect athletes of old.
Sponsors (godparents) represent the Church community, into which the baptized is being initiated.