Wikipedia
Helvidius (sometimes Helvetius) was the author of a work written prior to 383 against the belief in the perpetual virginity of Mary. Helvidius maintained that the biblical mention of "sisters" and "brothers" of the Lord constitutes solid evidence that Mary had normal marital relations with Joseph and additional children after the miraculous conception and birth of Jesus. He supported his opinion by the writings of Tertullian and Victorinus.
Jerome, in reply, wrote a treatise known under the title The Perpetual Virginity of Blessed Mary, where he vigorously takes the other side, and argues that the "sisters" and "brothers" spoken of were either step-brothers, children of Joseph by a former marriage (cf. Protoevangelium of James), or first cousins, children of Mary's relative/relation/kinswoman Elizabeth and siblings of John the Baptist. When Jerome wrote this treatise both he and Helvidius were in Rome, and Damasus was Roman bishop. The only contemporary notice preserved of Helvidius is contained in Jerome's treatise.