He was the eldest of six children of Lawrence Sim and Monica Yeo,[11] and his grandparents were the first Catholics in their village, with the Church's history in Brunei documented most prominently from the 19th century onwards.

Returning to Brunei, he rediscovered his Catholic faith and grew close to a charismatic group that downplayed doctrine and emphasised personal relationships with God.

[28] Three years later in 2001, Sim also attended the ad limina visit to Pope John Paul II by prelates from the Bishops' Conference of Malaysia, Singapore and Brunei.

[45][46] On 15 July 2014, Sim backed calls made by Brunei's reigning sultan to the country to pray for peace in Palestine amid rising tensions.

[47] Sim celebrated his 10th episcopal anniversary and coincidentally his silver jubilee of ordination to the priesthood on 21 January 2015 with a commemorative Mass at the Church of Our Lady of the Assumption in Bandar Seri Begawan.

[54][55] During Christmas of 2018, Sim spoke to local media in Brunei as thousands reportedly attended prayer services in the capital city, stating that Christians have time to find a "deeper purpose" for the occasion, and need to go "back to simplicity" to capture the day's true spirit.

[64] Sim continued to credit his charismatic roots as the basis of his faith, allowing that the movement has been appropriately "domesticated, maybe for good reason".

[72] In spite of a quietened coverage of his elevation as he did not attend the consistory, Sim still recognised his appointment to the cardinalate as being a sign of worldwide recognition of the nation's Christian community in general.

[75] During his final address to his congregation in May that year, Sim urged the People of God in Brunei not to "be spectators" in his absence, but to make visible their contributions of "time, talent and treasure.

[78] His passing received coverage in local state-influenced and independent news media and online forums, as well as Catholic outlets throughout the world.

[81][82] Speaking on Sim's death, the Archbishop of Kuala Lumpur Julian Leow said, "Brunei has lost one of her illustrious sons, the Bishops' Conference, a dear brother and the Church, a prince.

"[83] Archbishop of Kota Kinabalu John Wong said, "Cardinal Sim left an unforgettable mark in his untiring and devoted service to the Church, and for his constant advocacy of dialogue and peace for all people in Brunei.

[86] A wake (ceremony) was held for the late cardinal at the Church of Our Lady of Assumption, the parish which housed his cathedra in Brunei's capital, on 14 June 2021 as his body lay in state, enabling mourners to pay their respects.

[87] A requiem was held the next day, 15 June, at 9 AM and his body was driven through Brunei to all other parishes for the congregations to pay their last respects, before being buried in Kuala Belait.

[88][89] Among those in attendance at the funeral service were members of the diplomatic corps in Brunei, as well as representatives from other Christian churches and prominent local community figures.

[94] Sim was among the contributing theologians and bishops of a youth edition release of the New Testament intended for the young peoples of Asia, which was launched at the FABC general conference in Thailand on 21 October 2022.