He listened, was asked to remain a fortnight at the station, and soon had given his whole soul to the faith.
He was baptized by the name of Nyana Pracasam, or Spiritual Light, and became a catechist.

His father and mother were likewise led to Christianity by him, but the Roman Catholics, having begun his conversion, considered that they had a right to him, and on one occasion, when he was found reading to a sick relative, probably a member of their Church, he was severely beaten, and was rescued by the heathen neighbours when nearly killed.
Swartz seems to have regarded the Roman Catholics as in almost as much need of reconversion as the Hindoos and Mahometans; and as in those days their Church shared in that universal religious torpor that had crept over the world, it is most likely that he found them in a very debased condition.
With the Mahometans he had some success, though he found, like all other missionaries, that their faith, being rather a heresy than a paganism, had truth enough in it to be much harder to deal with than the Hindoo polytheism.

Besides, they accepted the Persian proverb, "Every time a man argues, he loses a drop of blood from his liver." He was impeded also by the want of a Persian translation of the entire Bible, having no more than the Gospels to give the inquirers, and these badly translated; and with Mahometans the want of the real history of the Patriarchs was very serious.

Some, however, were convinced and baptized, though by far the greater number of his converts were Hindoos.
In 1776, a coadjutor, either German or Danish-trained, named Christian Pohle, joined him at Trichinopoly, and thus he became free to reside more constantly at Tanjore, where the Rajah always protected him, though continually fluctuating in feeling towards Christianity, according to the influences of his ministers and the Brahmins who surrounded him, and the too frequent offences given by the godless officers of the European garrison which was stationed in the fort.
Mr.Swartz was anxiously soliciting for means to build a church for the use of this garrison, when he was summoned to Madras, to the governor, Sir Thomas Rumbold, who promised him a grant for his church; but, at the same time, informed him that he was to be sent on a mission to visit the formidable Hyder Ali in Mysore, in order to judge how far his intentions towards the English were pacific.

He was selected for the purpose on account of his perfect knowledge of Hindostanee, the simplicity of his manner of travelling, and his perfect immunity from any of the ordinary influences of interest or ambition; and he undertook it, as he tells the Society for Promoting Christian Knowledge, because he regarded it as conducing to peace, as opening fresh doors to the Gospel, and as a token of gratitude to the Honourable Company for kindness he had received; "but at the same time," he says, "I resolved to keep my hands undefiled from any presents, by which determination the Lord enabled me to abide, so that I have not accepted a single farthing save my travelling expenses." On the 1st of July, 1779, he set out from Trichinopoly on this journey, taking one of his catechists, named Sattianadem, with him.