In 1808, Dr Thomson was removed to the East church of Perth, where he laboured, assiduously and successfully, till the spring of 1810, when he received a presentation from the magistrates and council of Edinburgh to the New Grey Friars’ church in that city. He was now in a situation, where his singular talents could be fully appreciated, and where they had a field wide enough for their exercise: of these advantages he did not fail to avail himself. He applied himself to the discharge of his sacred duties with redoubled ardour, and with a vigour and activity both of body and mind, that at once procured him an extraordinary share of public admiration. His powerful eloquence and fearless charactor, pointed him out as no ordinary man, and made an impression on the public mind, which has but few parallels in the history of ministerial labours. Indefatigable and zealous, in a singular degree, he left no hour unemployed, and no means untried, to forward the good work in which he was engaged. He laboured incessantly; and such was the vigour and grasp of his comprehensive mind, and the versatility, as well as brilliancy of his talents, that he could, at one and the same time, bring the most various and wholly different means, to bear upon the one great end which he had in view, the spiritual and temporal happiness of mankind. To the discussion of every variety of subject within the sphere of his calling, he came alike prepared, and on each shed the strong light of his powerful intellect, exciting the admiration of all who heard him, by his manly eloquence, and convincing most, it is to be hoped, by the force of his reasoning.

Among the other means to which Dr Thomson had recourse to promote the interests of religion, was the publication of a periodical work, entitled "The Christian Instructor." This work he commenced, with the assistance of several of his clerical brethren, a few months after his settlement in Edinburgh; and for many years he discharged the duties of its editor, besides contributing largely to the work itself. It is almost unnecessary to add, after what has been said of Dr Thomson, that the "Christian Instructor" is a work of singular merit, and, altogether, perhaps, one of the ablest of the kind which the cause of Christianity has produced.

Dr Thomson’s literary labours were not, however, confined at this period to the "Christian Instructor." He contributed, besides, many valuable articles to the Edinburgh Encyclopedia; all of which are distinguished by that nervous style and accuracy of conception, which so peculiarly belonged to their author.

The extraordinary merits of Dr Thomson had early forced themselves on the public notice; but they were now become so obvious and incontestable, as to engross a very large share of the public attention, and to form a subject of its consideration. The result of this general feeling was, his appointment to St George’s church, which took place on the 16th of June, 1814; one of the most important and dignified charges in the church of Scotland. In this conspicuous situation, he rapidly extended his reputation, and increased the number of his friends; and, ultimately, acquired an influence over his congregation, composed of the most influential persons in the metropolis, which few preachers have ever enjoyed. Previously to his appointment to St George’s, Dr Thomson had not been in the habit of writing out his discourses. He trusted to the natural promptness with which his ideas presented and arranged themselves, and to the remarkable fluency of expression with which he was gifted; and these did not fail him: but he now thought it advisable, as he was to preach to a more refined class of persons, to secure more correctness for his discourses, by committing them to paper, before delivering them from the pulpit. And in the pursuance of this resolution, he weekly composed and wrote two sermons, and this in the midst of other avocations, which alone would have occupied all the time of any man of less bodily and mental activity than he was possessed of.

To the ordinary duties of the Sunday, Dr Thomson added the practice of catechising the young persons of his congregation, devoting to this exercise the interval between the forenoon and afternoon services. He also held week-day meetings in the church, for the purpose of instructing in the principles of religion, as they are taught in the Shorter Catechism; and, to complete the system of moral and religious culture, which his unwearying zeal had planned out, he instituted a week-day school, for the benefit of those of his young parishioners whose circumstances either prevented their attending church, or rendered a greater extent of tuition necessary than he could afford to bestow on Sunday. But he did still more than merely institute this little seminary. He compiled suitable books for the different classes it comprised, and crowned the good work, by acting himself as their teacher,—as the teacher of the poorest and humblest of his flock.

With all this devotion to the higher and more important duties of his sacred office, Dr Thomson did not neglect those of a minor character. Amongst these, church music had an especial share of his attention. Together with his other rare endowments, he possessed an exquisite ear and taste for music, and not only introduced an improved psalmody into the Scottish church, but added to it several eminently beautiful compositions of his own. Admirable as Dr Thomson was in all his relations to his flock, he was in none more so, than in that of the personal friend, the soother of affliction, and the alleviator of domestic misery. His private labours of this kind were very great, and eminently successful. His presence never failed to excite a new feeling of animation, nor his words to inspire hope. To the sick and the bereaved his peculiarly acceptable; for his manner and his language were kind, and soothing, and conciliating, in a remarkable degree: and, although these could not always lessen pain, they never missed of reconciling the sufferer to that which was inevitable.

Besides thus faithfully and laboriously discharging the various important duties of his office, Dr Thomson took an active part in all the church judicatories of which he was a member. In these, his singular talents and high character, as might be expected, always secured for him the first place, and at length acquired for him the distinction, conceded silently but spontaneously, of being considered the leader of the evangelical party in the church to which he had attached himself. Amongst the other characteristics of that party, was a strong feeling of hostility to the system of patronage; and to this feeling Dr Thomson gave utterance in the General Assembly, on several occasions, in a strain of eloquence, and with a power of reasoning, that will not soon be forgotten.

Although a zealous member of the church of Scotland, and strongly attached to her institutions, Dr Thomson’s liberal and enlightened mind kept him entirely aloof from anything approaching to bigotry. With dissenters of all descriptions he maintained a friendly understanding. He made every allowance for difference of opinion on points of comparatively inferior importance; and, when he was satisfied that a genuine spirit of Christianity existed, never allowed such difference of opinion to disturb that harmony which he wisely and benevolently conceived ought to exist between those who, after all, laboured in the same vineyard, and to obtain the same end.

Ever ready to lend his powerful aid to all rational schemes for promoting the interests of religion and extending its sacred influence, he eagerly enrolled himself amongst the supporters of the British and Foreign Bible Society; and while that society adhered to the principles which were laid down at its institution, he continued to take a warm interest in its affairs, and laboured with tongue and pen to secure success to its efforts. On the departure, however, of this society from one of the leading conditions by which it was understood it should be regulated, namely, that the copies of the Bible which it issued, should be purely scriptural, and unaccompanied by note or comment of any kind; Dr Thomson felt himself called upon, as a minister of the gospel, not only to withdraw his support from it, but to oppose, by every means in his power, the continuance of a system so injurious to the best interests of religion. Into the well known controversy which ensued, and which has been called "the Apocrypha Controversy," he entered with all his characteristic zeal; and so effectually employed his powerful talents during its progress, that whatever cause they may have found for rejoicing in the issue, could find but little in the circumstance of having provoked his resentment.

The last great public effort of Dr Thomson was in behalf of the slaves in our West India colonies; and, in the prosecution of this humane and philanthropic work, he, on several occasions, made displays of oratory, which have been seldom equalled, and still seldomer surpassed. He demanded immediate emancipation, and supported this demand with an eloquence and power of reasoning, which were altogether overpowering.