Simply, we believe it is the only biblically valid mode of baptism so we do not believe someone has been biblically baptized without being immersed. We require biblical baptism for membership because it fits the pattern of the book of Acts, it is how someone makes a public profession of faith in Christ (which is a necessity for church membership), and it expresses the symbolism of a person being baptized into the universal body of Christ by the Holy Spirit at the moment of conversion.