Substantive due process guarantees that no person's fundamental constitutional rights will be infringed upon. This has to do with the way the laws are enforced and carried out.
The substantive due process law in the United States Constitution lays down rules for the states when it comes to a person receiving due process. Adopted in 1868, it says that "nor shall any state deprive any person of life, liberty, or property, without due process of law." Basically, this clause is saying that the law cannot come in and take away a person's freedoms without giving him a fair trial first. Freedoms protected under this clause include the freedom of religion and freedom of speech.