Answer
The Supreme Court has used the Fourteenth Amendment Due Process and Equal Protection Clauses to make the states adopt the Bill of Rights, which originally applied only to the federal government.
The statement, "The Supreme Court has nationalized the Bill of Rights" is a bit misleading, and not entirely true, because the Court uses a process of selective incorporation to enforce individual clauses within the Bill of Rights as need arises. There are still portions of the Bill of Rights that do not apply to the states (for example, the Second and Seventh Amendments are not incorporated, nor are small portions of several other Amendments).
Explanation
The Bill of Rights is the first ten Amendments to the Constitution, most of which address people's fundamental rights in relationship to the government and the law. When the Constitution was first created, these Amendments only applied to the federal government; the states enjoyed a greater degree of independence, and weren't required to offer the same protections. In the 18th and 19th-centuries, the Supreme Court typically deferred to state laws and constitutions in legal disputes, even if the state law lead to injustice and the case would have been settled differently under the US Constitution.
When Congress and the states ratified the Fourteenth Amendment in 1868, it provided the Supreme Court with a tool it would later use selectively to make the states comply with US Constitutional protections:
Fourteenth Amendment, Section 1
"All persons born or naturalized in the United States, and subject to the jurisdiction thereof, are citizens of the United States and the State wherein they reside. No State shall make or enforce any law which shall abridge the privileges or immunities of citizens of the United States; nor shall any State deprive any person of life, liberty, or property, without due process of law; nor deny to any person within its jurisdiction the equal protection of the laws."
Scholars usually cite the consolidated Slaughterhouse Cases, 83 US 36 (1873) as the first instance where the Supreme Court interpreted the action of the Fourteenth Amendment on the states, when it upheld Louisiana's right to create an exclusive monopoly of slaughterhouses for a limited number of companies. These companies employed laborers under conditions similar to those of slavery, but the employment was considered voluntary (despite the servitude being a product of limited options).
In the Slaughterhouse Cases, the Court decided "the main purpose of all the three last amendments [Thirteenth, Fourteenth and Fifteenth] was the freedom of the African race, the security and perpetuation of that freedom, and their protection from the oppressions of the white men who had formerly held them in slavery."
The Court declined to construe the meaning of the Fourteenth Amendment as a mandate on the states to respect and adhere to principles in the US Constitution, but instead emphasized the separateness of the federal and state governments, upholding state sovereignty.
The balance of the decision in Slaughterhouse makes clear the Court's opinion that the Thirteenth, Fourteenth and Fifteenth Amendments were intended exclusively to abolish slavery and confer citizenship and civil rights on African-Americans (which they acknowledged would extend to Mexicans, Chinese, and other oppressed groups), while maintaining the status quo with regard to everything else.
"But, however pervading this sentiment [regarding abolition of slavery], and however it may have contributed to the adoption of the amendments we have been considering, we do not see in those amendments any purpose to destroy the main features of the general system. Under the pressure of all the excited feeling growing out of the war, our statesmen have still believed that the existence of the State with powers for domestic and local government, including the regulation of civil rights the rights of person and of property was essential to the perfect working of our complex form of government, though they have thought proper to impose additional limitations on the States, and to confer additional power on that of the Nation.
"But whatever fluctuations may be seen in the history of public opinion on this subject during the period of our national existence, we think it will be found that this court, so far as its functions required, has always held with a steady and an even hand the balance between State and Federal power, and we trust that such may continue to be the history of its relation to that subject so long as it shall have duties to perform which demand of it a construction of the Constitution or of any of its parts."
This interpretation remained precedent until the Taft Court determined, in Gitlow v. New York, 268 US 652 (1925), that the Fourteenth Amendment Due Process Clause extended First Amendment protection of free speech and freedom of the press to all citizens, and that the states could enact legislation against "...utterances [that] present sufficient danger to the public peace and security of the State..." but could not prohibit expression of anti-government (or other) ideas that did not advocate or incite concrete, dangerous action.
This revolutionary reinterpretation of the Fourteenth Amendment instigated a trend toward applying individual clauses of Amendments within the Bill of Rights to the states, in response to Supreme Court cases challenging state laws infringing people's US Constitutional rights. The process described is called "selective incorporation," and has been used to impose most, but not all, of the Bill of Rights on the states.
For more information about selective incorporation see Related Questions, below.
This is a reference to the "due process" clause (section 1) of the separate 14th Amendment, which extends the authority of the Bill of Rights (first 10 amendments) to the exercise of governmental power by the states. Rather than the total incorporation of the Bill of Rights, the extension of their application has been done in selected cases over a long period of time.
"No State shall make or enforce any law which shall abridge the privileges or immunities of citizens of the United States; nor shall any State deprive any person of life, liberty, or property, without due process of law; nor deny to any person within its jurisdiction the equal protection of the laws."
The first ten Amendments were ratified together, reaching ratification by three-quarters of the States then comprising the Union, on December 15, 1791. Previously, Amendments I-X were considered the "Bill of Rights". Modernly, the Bill of Rights is considered only Amendments I-VIII, because only these Amendments describe individual rights. Amendment IX and Amendment X refer to collective, residual rights reserved to the People and to the States.
Bill or rights guarantees.
The First Amendment
The Bill of Rights.
The Bill of Rights
The 14th Amendment. Incorporation - the process by which court decisions have required the states to follow parts of the Bill of Rights based on the use or application of the 14th Amendment- which continued to occur gradually, up until the last incorporation case in 1969 (its also sometimes referred to as the "absorption' or the "nationalizing" of the Bill of Rights). -- This is what my book said. I hope that it helps!
The First Amendment
The first ten Amendments were ratified together, reaching ratification by three-quarters of the States then comprising the Union, on December 15, 1791. Previously, Amendments I-X were considered the "Bill of Rights". Modernly, the Bill of Rights is considered only Amendments I-VIII, because only these Amendments describe individual rights. Amendment IX and Amendment X refer to collective, residual rights reserved to the People and to the States.
the First Amendment
The bill of rights is the name for the first ten amendments to the Constitution of the United States of America. It defines the fundamental rights that American citizens have, such as freedom of speech and the right to bear arms.
Bill or rights guarantees.
The English Bill of Rights
The First Amendment
The Bill of Rights
The Virginia Declaration of Rights was a model for the Bill of Rights.
The traditional answer is that Amendment I through Amendment X, ratified by 1791, formed the Bill of Rights. More modern Constitutional scholarship holds that if one conceives of the Bill of Rights as a manifesto of individual rights, this is only comprised by Amendments I through VIII, as Amendment IX and Amendment X refer to residual rights reserved to the people and the States.
English Bill of Rights