According to Saint Paul, baptism has three elements:
The New Testament does not provide a precise ritual for the celebration of baptism. The only constant element is water (i.e., Acts 8:36–38).
At the end of the 1st century, baptism was performed in "living water" (streams or rivers). Early Christians preferred immersion although pouring was also accepted.
By the late 3rd and early 4th centuries, a ritual involving immersion in lots of water and anointing with oil was solidly established. The great baptisteries of Northern Africa and Southern Europe testify to the importance of the rite and the sacrament.
During the 6th and 7th centuries, the ritual declined as it gradually became privatized and minimized. Child baptism became the norm using a minimal amount of water.
The Second Vatican Council returned to the great baptismal period of the Church and reintroduced immersion and baptism of adults.
It establishes a vital union with Christ in the saving events of His life, His death, and His resurrection.
It involves the reception of the Holy Spirit.
It establishes the body of Christ, the assembly of Christians.