Since it's founding in 1949, the People's Republic of China has had four different constitutions. The first one was ratified in 1954, and laid out the guiding principles, as well as establishing state structures and citizen rights and obligations. The second constitution was ratified in 1975, with a third, amended version being ratified in 1978.
The current Constitution of the People's Republic of China was ratified in 1985. Each reflects the unique political and social conditions in China at the time of ratification.
Each constitution has an article that references religious belief.
"Citizens of the People's Republic of China enjoy freedom of religious belief."
During the Cultural Revolution, the constitution was cast aside and Mao ruled by decree. Towards the end of the Cultural Revolution, the Constitution was revived and revised.
"Citizens enjoy freedom of speech, correspondence, the press, assembly, association, procession, demonstration and the freedom to strike, and enjoy freedom to believe in religion and freedom not to believe in religion and to propagate atheism."
"Citizens enjoy freedom to believe in religion and freedom not to believe in religion and to propagate atheism."
"Citizens of the People's Republic of China enjoy freedom of religious belief. No state organ, public organization or individual may compel citizens to believe in, or not to believe in, any religion; nor may they discriminate against citizens who believe in, or do not believe in, any religion. The state protects normal religious activities. No one may make use of religion to engage in activities that disrupt public order, impair the health of citizens or interfere with the educational system of the state. Religious bodies and religious affairs are not subject to any foreign domination."
What all of these articles has in common is the notion of having a right to believe (or not) in religion.
Article 36 is actually the most comprehensive of all the articles in the protections that it seems to offer citizens. They can't be forced to believe in religion. They can't be forced not to believe in religion. Those who believe in religion cannot be discriminated against.
However, the provisions in the second half of the article seem to nullify, or at least place limits on, the freedom promised in the first half. After stating what freedoms citizens have, it basically says "but the state can set the boundaries around religion" (regulate). The state gets to define what religious activities are considered to be "normal." The state gets to decide what activities are considered "disruptive" to social order, impair the health of society, and interfere with the educational system. According to some Mainland legal scholars, then, the first half of the article protects religious freedom, while the second half restricts it. (Am I allowed to say, "nothing is as it seems" at this point?)
At the end of the day, however, it doesn't really matter all that much what the constitution says or doesn't say because in the Chinese legal system courts do not have the power of judicial review. In other words, they cannot consider the constitution when deciding cases. It's just not relevant.
As I mentioned earlier, for years we have often heard that the Chinese Constitution forbids the teaching of religion to children under the age of 18. The good news, as we can see, is that it isn't true. The Chinese Constitution does no such thing.
The bad news, however, is that such a stipulation does exist in "Document No. 19 The Basic Viewpoint and Policy on the Religious Question during Our Country's Socialist Period," which was promulgated by the State Council (cabinet) in 1982. This was a detailed document put out detailing how religion could/could not function in Chinese society. It states:
"The political power in a socialist state can in no way be used to promote any one religion, nor can it be used to forbid any one religion, as long as it is only a question of normal religious beliefs and practices. At the same time, religion will not be permitted to meddle in the administrative or juridical affairs of state, nor to intervene in the schools or public education. It will be absolutely forbidden to force anyone, particularly people under eighteen years of age, to become a member of a church, to become a Buddhist monk or nun, or to go to temples or monasteries to study Buddhist scripture."
In 2005, the State Council promulgated the "Regulation on Religions Affairs," in which it further clarified how religious affairs are to be managed in China. Interestingly, this time there is no mention of prohibitions for those under the age of 18.
"Citizens enjoy freedom of religious belief. No organization or individual may compel citizens to believe in, or not to believe in, any religion; nor may they discriminate against citizens who believe in any religion (hereinafter referred to as religious citizens) or citizens who do not believe in any religion (hereinafter referred to as non-religious citizens). Religious citizens and non-religious citizens shall respect each other and co-exist in harmony, and so shall citizens who believe in different religions."
"The State, in accordance with the law, protects normal religious activities, and safeguards the lawful rights and interests of religious bodies, sites for religious activities and religious citizens. Religious bodies, sites for religious activities and religious citizens shall abide by the Constitution, laws, regulations and rules, and safeguard unification of the country, unity of all nationalities and stability of society. No organization or individual may make use of religion to engage in activities that disrupt public order, impair health of citizens or interfere with the educational system of the State, or in other activities that harm State or public interests, or citizens' lawful rights and interests."
In other words, people can believe what they want, but the state reserves the right to set the boundaries within which their beliefs can be practiced.
For a detailed analysis of these regulations, I recommend an article titled "Semantic Analysis of Keywords in the Regulation of the Religious Affairs," by Zhang Shoudong, a Mainland scholar.
The Center on Religion and Chinese Society at Purdue University has a collection of English translations of government documents on religion in China.