Linguistically – baptism is the transliteration of the Greek “baptizo” which always means to plunge, submerge, immerse or dip. In New Testament times ‘baptizo’ was an ordinary everyday word, without overt religious significance. It was used in several ways; for example, to describe drawing water out of a well or dyeing a garment. Both of those situations require full immersion of either bucket or garment! Another Greek word “rantizo” means sprinkling but is never used in the New Testament of baptism.
Symbolically – immersion is a graphic portrayal of the reality of dying to sin and being raised to new life in Christ. The death-resurrection parallel is powerful – Christ physically died and bodily rose again. He even described his own impending death as a baptism he had to undergo! We die to self and rise again to new life. Down and Up. Submerge and Reemerge. Great stuff!
Theologically – baptism is closely identified with discipleship and is a physical mark of it. Matthew 28 identifies it as a major element of the Great Commission. The subject therefore must always be one who has started on the road, who is learning to follow and who is desiring to keep going. That is the only legitimate candidate for baptism. When the mark of the covenant (circumcision) was applied to the Old Testament people of God the Israelites, it was applied, not in anticipation of them becoming the children of God, but because they already were. God knows no other way. The mark of discipleship can only be applied to those who have started on that road – not because they potentially might at some undefined time in the future.
Practically – nothing more clearly portrays the gospel than the ordinances Christ gave to his church. The regular, ongoing, participatory ordinance of the Lords Supper and the once only, one time event ordinance of believers baptism. Both clearly portray the death and resurrection of Christ in easily understood pictures and symbols. Both need to be regular features of local church life, because both prioritize the gospel message and explain it to the eyes of a watching unbelieving world.