Adam Winkler, professor of constitutional law at the UCLA School of Law, explains the constitution as the founding document which establishes the federal government, relationship between the branches of the government, and the relationship between the federal government and the states. It also establishes a set of individual rights that you can claim to prevent the government from doing certain things.
When the Constitution was written, gun ownership was not thought of as means of protection for a family. Gunpowder was very explosive so you would not want to actually store it in your home, and guns were very basic, where it would take a minute just to reload it after shooting. If someone were to break into your house at the time, you would not use a gun as first means of defence.
As times went by, however, the social norms around gun ownership have changed and the focus on self-defence have become more prevalent, to the point where the Supreme Court justices are now interpreting the second amendment in a way in which it was perhaps not intended.
Americans tend to revere the constitution and use it as a central point in political debate because the language of the constitution is vague and opens room for interpretation. Freedom of speech, equal protection of the law ...etc. Almost anything can fit in some shape of form under one of the principles in the Constitution, but we cannot be present for the original writing in order to confirm the exact original intend.