The Belgian Constitution of 1831 is known as a liberal constitution, and with good reason. It introduced several freedoms – of religion, education and association, among others – that under the Dutch king William I (1815-1830) had been more or less strongly limited. As a result, it left social forces free to organise themselves. But by doing this, the Constitution of 1831 in retrospect also opened the way to frictions among contrasting social views. This article reconstructs the intellectual background and organisational strategies of the Catholic Church, one of the two major forces behind Belgium’s independence, within the frame of the large constitutional freedom it was granted. How did Belgian prelates relate to contemporary international and theological evolutions, like the excommunication in 1832 of Lamennais’s Liberal Catholicism, one of the most influential sources of inspiration for Catholic revolutionaries in 1830? How did they manage to preserve the dominant position of the Church in the educational sector against the growing mobilisation of Liberal forces? And, last but not least, how did they attempt to shape their pastoral activities among a linguistically plural population? The compromise of 1831 was clearly made possible by the mainly terminological convergence towards freedom amongst Liberals and Catholics. This does not mean, though, that the so-called Unionist consensus of the 1830s remained free from frictions. On one hand, the constitution and its positive effects for the Church had to be unlinked from Lamennais’s Liberal Catholicism in order to avoid a showdown between the episcopate and the state. On the other, the ambiguity of certain constitutional articles on matters of shared interests between Church and State, especially article 17 on education, reveals that a certain degree of ambiguity about liberty was in itself a source of possible conflict whenever sensitive legal articles needed to be translated into the real constitution. The outcome of negotiations varied according to the power relations of the moment. It is no surprise, therefore, that the defence of the Church’s interests in the aftermath of Unionism inspired conservative and reactionary clergy to adopt a more assertive strategy, as anticipated already in the 1840s in the struggle against a new – more Dutch – spelling for the Flemish language.