In 1814 he was approached to accept the ministry at Tron, Glasgow. He was not sure about leaving Kilmany, indicating that he would do nothing himself to secure this new position, he would leave it to God to bring it about or block it. There was much opposition to his appointment from the Moderate party, as they tried to persuade city councilors, who had the patronage over the appointment, to choose their man. However, the Evangelical party won, with Chalmers getting 15 out of 29 votes. He was still not sure about accepting the position. Obviously, he had a loyalty towards the people of Kilmany, but he was also worried that he would not be able to live up to his reputation in the big city and that he would be tied down with superfluous duties. In the end he put his worries aside, trusting that this was God’s will.
Chalmers began his ministry in Glasgow on July 21st 1815. His reputation had gone before him; the church seats all being sold in no time. At this time the population of Glasgow was around 120,000, around three times what it was thirty-five years earlier. It had grown through the proliferation of manufacturing, particularly cotton.
His worries about superfluous duties were not groundless. The people demanded clergy to be at every committee meeting, procession etc; he even had to decide whether a gutter should be covered. In the end he just refused to attend such occasions. Another problem was his fame. Everyone wanted to meet him and then invite him to social gatherings. He dealt with this problem in the same way, saying no. Something else he was burdened with was having to re-organise his parish. In order to free up the minister there were deacons and elders to carry out some of his duties, but their work had reduced to practically nothing. He began by visiting his parishioners as he did at Kilmany. He could not spend much time with each one, but he did what he could, and invited them to a week-night service that he would put on in a local building. There was little opportunity for the poor to attend church, because they could not afford the seat rents or the smart clothes that people wore to church. He later commented, “I can say that out of 10,000 individuals whom I visited, there were not six by whom I was rudely or disrespectfully treated.” However, it took him a year to tour his parish, so the effectiveness of this work was very limited. To solve the problem he appointed twelve new elders who had the time and resources to visit the parishioners. Apart from speaking to people about Christian issues, they were also to identify who had financial or other needs, and they were to recruit other, like-minded men, who had time and money, to help them in the work. As they found people in need, they would use their own funds to give relief, but in extreme circumstances they would be referred for institutional relief. The aim was to ensure that the spiritual and temporal needs of the parishioners were looked after. Quite a radical proposition in a large urban parish.
Chalmers then tackled education, or lack of, in the parish. He had discovered that only 100 children went to school in his parish. He started Sabbath-schools which only took students from his parish. He appointed teachers who visited the houses in his area to ask each child to come to the school. Soon there were 40 teachers and 1,200 children were attending classes. The model that Chalmers created, only allowing children from the parish to attend and the teachers visiting homes, was duplicated in other cities very successfully. His idea of the parish being a community was beginning to be understood. His idea of helping to build up the poor so that they could help themselves rather than rely on handouts; was quite revolutionary.
While Chalmers was spending so much time in his parish, there were murmurings from his congregation who wanted him to spend more time with them. When he told them that he was thinking of moving to Stirling, they got back into line, even giving him an assistant.
Chalmers’ reputation as a preacher reached new heights in Glasgow. He soon became known as the greatest preacher in Scotland. His greatest success was probably a series of six talks, known as his ‘Astronomical Discourses,’ that he preached at the Thursday lecture in his church, which was rotated with eight city ministers. He finished them towards the end of 1816, and on their being published early in 1817, 6,000 were sold in a month and nearly 20,000 within the year. No sermons had sold like this before, and Chalmers was invited to London to preach the anniversary sermon for the London Missionary Society. He spoke four times while in London to vast crowds, with many being turned away for lack of room. William Wilberforce noted, “All the world wild about Dr Chalmers.” (He had received a Doctorate from Glasgow University.) His friend who came to England with him wrote, "I write under the nervousness of having heard and witnessed the most astonishing display of human talent that perhaps ever commanded sight or hearing. Dr Chalmers has just finished the discourse before the Missionary Society. All my expectations were overwhelmed in the triumph of it.” Even Canning, who was one of his hearers, and who was melted into tears by his sermon for the Hibernian Society, declared that, "notwithstanding the northern accent and unpolished manner of the speaker, he had never been so arrested by any kind of oratory." He met all sorts of dignitaries and formed friendships with Wilberforce and some of his friends who were doing such wonderful work to do with the slave trade and social reform.
Because his church on a Sunday morning was full of the middle and upper classes who could afford the seat rates, he started an evening service for the poorer members of his parish. These meeting also tended to be full. He then set about the question of poverty. He had long been against the legal poor relief that the poor had to rely on, as it was demeaning, and it encouraged the poor not to get themselves out of their situation, as they could rely on the poor relief. His proposal was to stop any more people getting assessed poor relief. New people needing help would get it from the collections in church, the elders would investigate applicants to ensure they were genuine cases and the wealthy would be encouraged to help the less severe cases. In this way those receiving poor relief would gradually die off or regain their independence until nobody was receiving it. All the money saved fro the poor relief would be used to build more schools and churches (he proposed 30 more in Glasgow). The extra churches would mean that the parishes would reduce in size from 10,000 to 3,000, which would make it easier to establish the communal spirit Chalmers advocated, such as he developed in Kilmany. The communal Christian ethos would mean everyone would be aware of their neighbours situation, and their love f Christ would encourage them to help those in need.
There were many critics Chalmers’ proposals, including leaders of the Evangelical party, but he found an influential ally in someone on the Town Council who persuaded the council to give a church they were building in Chalmers’ parish as an experiment for his ideas. They agreed to several conditions so that the experiment could be carried out effectively. However, Chalmers’ old insecurities rose up again, so he wrote to the Council asking them to confirm again their promises. On not receiving a reply he thought that they had gone back on their word, so he spoke to the Edinburgh Whigs to see if they would propose him to the vacant Chair of Natural Philosophy. When the news got out there was an uproar everywhere as it looked like he was betraying his church, the Town Council and the public. As a result his name was withdrawn. In the end the Town Council delivered their promises, but Chalmers’ reputation was greatly damaged.
St John’s, was opened on September 26th, 1819, and was crowded by its new parishioners.