During the six years which elapsed between the consecration and the beginning of the great rebellion, Leslie was chiefly engaged in warfare with the Ulster presbyterians of Scottish race, becoming a member of the high commission court in February 1636. In May he preached at Newtownards on the death of the first Viscount Montgomery, and in July he held his primary visitation at Lisburn. Five ministers, including Lord Clandeboye's nephew, James Hamilton, there refused subscription to the new canons. Being urged by Bramhall to extreme measures, he preached at Belfast on 10 Aug. on the significant text, 'If he neglect to hear the church, let him be unto thee as an heathen man and a publican.' This sermon, in defence of Anglican orders and of kneeling at the communion, was printed in the following year as 'A Treatise of the Authority of the Church.' It is dedicated in panegyrical strain to Wentworth, who is called 'Instaurator Ecclesiae Hibernicae.' Leslie says that presbyterianism made most progress among women. On the day after the sermon a disputation in strict syllogistic form took place between the bishop and Hamilton as spokesman for his brethren. Leslie, unlike Bramhall, played his part like a gentleman. The result was that the five ministers were deposed, the bishop expressing his sorrow at having to proceed so far (Reid, i. App. iv.) Leslie was now regarded as a champion of Laudian episcopacy, and works by John Corbet (1603–1641) [q. v.] were attributed to him.
The outbreak in Scotland gave great confidence to the presbyterians of Ulster, and on 26 Sept. 1638 Leslie preached at Lisburn against the solemn league and covenant. A Latin version of this sermon, entitled 'Examen Conjurationis Scoticae', was published by his chaplain, James Portus, in 1639. Along with his namesake, John Leslie (1571–1671) [q. v.], Leslie was one of those who signed the petition resulting in the proclamation of 1639. This imposed the black oath, by which every Scot, of either sex and of any age over sixteen, might be made to renounce the covenant and to swear unquestioning obedience to all the king's commands (Strafford, Letters, ii. 345). The bishop was active in the proceedings against Robert Adeir of Ballymena, who had subscribed the covenant as a Scottish laird, and whose Irish estate was confiscated by Wentworth (ib., ii. 219, 226). Leslie complains that his communications with Scotland were interrupted, that the very churchwardens had joined the general league against him, and that the contemners of his jurisdiction were 'more in number than would fill all the gaols in Ireland.' He even believed his life to be in danger. A viceregal commission giving him summary power of imprisoning those who refused to appear in his court furnished the ninth article of Strafford's impeachment.
At the beginning of 1640 Leslie had a long illness from which he hardly expected to recover, and was unable to attend the parliament which met on 16 March; but even from his sick bed he wrote a memorandum for Strafford as to the best means of increasing the royal revenues in Ulster (ib. ii. 302). In the following month Strafford left Ireland for ever, and the system which he had laboriously built up soon began to crumble away.
The rising of the Irish catholics followed. On 23 Oct. 1641 Leslie was at Lisburn, whence he wrote two letters to Lord Montgomery for help. At six in the evening he reported that Charlemont had fallen, and that Sir Phelim O'Neill [q. v.] and his horde were at Tanderagee. Four hours later he had heard that Newry was taken (Aphorismical Discovery, i. 384). Lisburn at once became the chief refuge of the Antrim protestants,l and fifteen hundred men were at once assembled in and about the bishop's house (Reid, i. 319). He did not take the field himself, but his sons James and William both led companies. In June 1643 he deposed to the total loss of his property by the war, and he withdrew to England; for north-east Ulster had escaped O'Neill only to fall into the hands of the covenanters. He preached at Oxford on the Fast-day 9 Feb. 1644, before a great many members of the House of Commons, and again on 27 March before some peers and many of the lower house. Afterwards he joined Ormonde in Dublin, and was one of eight Anglican prelates who, on 2 Aug. 1645, there refused to forgo the power of the keys over Roman catholics (Irish Confederation, v. 40). Just twelve months later he was one of thirteen bishops who, with seventy-seven other clergymen, presented an address of gratitude to the lord-lieutenant (Appendix to Carte, No. 471). Ormonde surrendered Dublin to the parliament in 1647, and Leslie went abroad either before or just after the king's execution. In June 1649 he preached at Breda on the royal martyrdom before Charles II and the Princess of Orange. The sermon was printed at the Hague and translated into Dutch, and there is an instructive English reprint of eighteenth-century date. In drawing an elaborate parallel between Charles I and Jesus, Leslie compares presbyterianism and independency to the two thieves between whom Christ was crucified.
The next ten years were doubtless spent
