At sixteen years of age, while visiting Paris, he converted to Catholicism and subsequently went to study in Rome, where in 1642 he was ordained as a Catholic priest. Three years later, he became a Jesuit.
Arrest and execution
He was arrested in November 1678, at Llantarnam in Monmouthshire, and condemned as a Roman Catholic priest and for saying Catholic masses, at the Assizes in Monmouth in March 1679. Like St John Wall and St John Kemble, he was then sent to London to be examined by Titus Oates (the originator of the Popish Plot) and others.
He was brought for trial at the Lenten Assizes in Monmouth on 16 March 1679. He was brought to the bar on a charge of High Treason – for having become a Catholic priest and then remaining in England.
He pleaded not guilty to the charge of being an accessory to the Popish Plot, but five or six witnesses claimed they had seen him say Mass and perform other priestly duties. For this Lewis was found guilty and sentenced to death by Sir Robert Atkins. The condemned priest was brought to Newgate Prison in London with John Kemble (Herefordshire) and questioned about the "plot". Oates, William Bedloe, Dugdale and Prace were unable to prove anything against him. Lord Shaftesbury advised him that if he gave evidence about the "plot" or renounced his Catholic faith, that his life would be spared and he would be greatly rewarded. Lewis said in his dying speech, "discover the plot I could not, as I knew of none; and conform I would not, for it was against my conscience". He was returned to Usk and waited for three months for his call to death by execution.
He was finally brought back to Usk in Monmouthshire for his execution, and was hanged on 27 August 1679. After the Titus Oates affair (1679–80), the remaining Welsh-speaking Catholic clergy were either executed or exiled.