This can be stated in active form. Alternate translation: "Philip had only baptized the Samaritan believers" (See: [[rc://en/ta/man/translate/figs-activepassive]])
Here "name" represents authority, and being baptized into his name represents being baptized in order to be under his authority. Alternate translation: "they had only been baptized to become disciples of the Lord Jesus" (See: [[rc://en/ta/man/translate/figs-metonymy]])