[3] He then moved to Rome and earned a licentiate in dogmatic theology from the Pontifical University of Saint Thomas Aquinas in 1983.

[2] He was parish vicar of St. Francis Xavier in Malacca in 1984; professor of dogmatic theology and spiritual director at the major seminary of Penang, chaplain of university students in Penang, administrator of the Church of the Immaculate Conception in Paulau Ticus-Penang and pastoral coordinator from 1985 to 1988;[3] vicar general of Malacca-Johor from 1988 to 2001;[2] pastor of the Cathedral in Malacca from 1988 to 2004; diocesan administrator of Malacca-Johor in 2002-2003; parish priest of St. Louis in Kluang from 2004 to 2007; parish priest of the Immaculate Conception from 2007 to 2011; and pastor of Christ the King in Kulai in 2011–2012.

[3][2] He received his episcopal consecration on 20 August 2012 at St Anne’s Church in Bukit Mertajam[4] from Archbishop Murphy Nicholas Xavier Pakiam, with Bishops Antony Selvanayagam and Paul Tan Chee Ing serving as co-consecrators.

[5] On 9 July 2023, Pope Francis announced he planned to make Sebastian a cardinal at a consistory scheduled for 30 September.

At that consistory, Pope Francis made him a cardinal priest, assigning him the title of Santa Maria Causa Nostrae Laetitiae.