On April 9, 1909, the nine-year-old Sung witnessed an "unprecedented revival" during a Good Friday sermon preached by Pastor Lim Hongban in Hinghwa which left a lasting impression.

Despite the array of career opportunities in front of him, Sung believed that he was called by God to commit himself to work for Jesus Christ.

Its professors publicly called for Christian unity, lobbied for international peace and discouraged the use of "imperialist force" in the various crises that were occurring worldwide.

[6] While still at the seminary, Sung set aside much of his Christian faith and chanted Buddhist sutras, studied mysticism, investigated the theosophic societies and other sects that were found in New York.

'"[9] After this experience John Sung now felt that he possessed more strength and enthusiastically preached the Gospel to people he met.

It was such a drastic change in the man that his fellow liberal theology students reckoned that he had gone out of his right mind, and the seminary authorities confined him in an insane asylum, where he stayed for 193 days.

The Chinese Consulate managed to arrange for Sung's release and he returned to China in November 1927, without graduating from Union Theological Seminary.

Before stepping on Chinese soil after an almost eight-year absence, he threw all his academic awards into the sea, only keeping the doctorate diploma for his father.

Sung was sometimes so excited that he often jumped onto the pulpit to preach; in the middle of the sermon he would always sing a hymn for up to twenty or thirty minutes.

(This was a play on words in the original Chinese, used to convey the idea that pursuing money will not bring you eternal life.)

[10] His missionary work spread beyond China and he was actively preaching to overseas Chinese population in Southeast Asia.

[11] Thai church leaders Suk Phongnoi and Boon Mark Gittisarn served as Sung's translators at different times during his evangelistic tours in Thailand.

It is claimed that John Sung was the most influential Chinese evangelist during the 1930s and his ministry had a profound impact on Christianity, especially in the Chinese-speaking world in China and Southeast Asia.

Countless Chinese were converted and revived through his ministry and they would play a significant role in the growth of Christianity in the region.

[15][16] A confession from a woman in Indonesia when John Sung entered the church from the door, sick people nearby suddenly healed as he walked forward to the altar.

It happened in a service in the Portugeesch buitenkerk (known today as Gereja Sion) in Jakarta, Indonesia, in the 1930s (it was Batavia, Dutch Indies, on that time).

[citation needed] There were numerous reports of other apparently miraculous healings taking place in various areas under Sung's ministry, such as those in China being repeated in Thailand.