In 1640, as a proctor in convocation, he urged the necessity of a new edition of the Welsh Bible, none having been published since that of Bishop Parry in 1620.
He described himself as an 'episcopal presbyterian,’ and waged a fierce war against independents and other sectaries, defended the parochial system, and boasted that 'he had withstood popery both by writing and preaching as much as any minister in Wales.'
'A Bold Challenge of an Itinerant Preacher (Vavasor Powell) modestly answered by a Local Minister to whom the same was sent and delivered; and severall Letters thereupon' [in Latin], London, 1652, 4to.
'A Welsh Narrative corrected and taught to speak true English and some Latine, or, Animadversions on an imperfect relation in the "Perfect Diurnall," Numb.
He was allowed to retain his old preferments in commendam, as well as the archdeaconry and the sinecure rectory of Llanrhaiadr yn Mochnant, as the revenues of his see were 'insufficient to maintain the state of a prelate' (Cal.
says that he 'not only concurred effectually in drawing up the Act of Uniformity, but the form of baptism for those of riper years was of his composing.'
The main work of Griffith's bishopric was to restore order and uniformity and look after the fabrics of the churches.
Besides the pamphlets against Powell, Griffith wrote some 'Plain Discourses on the Lord's Supper,’ published at Oxford in 1684.
In 1685 there was also printed at Oxford 'Gweddi'r-Arglwydd wedi ei hegluro, mewn amrŷw ymadroddion, neu bregethau byrbion, o waith G. Griffith diweddar escob Llanelwy.'
Some writings by him are preserved in manuscript in the collection of Miss Conway Griffiths, his descendant (Hist.