Baptism is done with physcial water, but isin’t washing away sins a spiritual thing? How can physcial stuff wash sins away from the spirit?
Baptism is a sacrament, and like all sacraments works through the action of the Holy Spirit by what it signifies. The pouring of water isn’t what scrubs the sinner free of sin, it is the action of the Holy Spirit, which action is mediated by the action of pouring water.
Hence sacraments are sometimes called “symbols that cause what they symbolize.”
The Sacraments are outward signs, instituted by Christ, that give grace. It is the sanctifying grace that accomplishes what the Sacrament signifies.
Primarily because God ordained the use of these visible signs to effect invisible realities. It only works because God has bound the use of natural water to the remission of sins.
It is not only water. A sacrament is made up of two parts: form and matter. The matter of baptism is water, the form is the words: I baptize you in the name of the Father, the Son and the Holy Spirit. One must use both of these at the same time, for the sacrament to be valid. The water symbolizes that which happens on a spiritual level. As water washes and cleans, in the material world, so the sacrament of Baptism washes all sins of the baptized person.
The sacraments are an external sign, instituted by Jesus Christ, that impart grace.
Very simply put… Because God said so.