It doesn't explicitly say that states can't secede. However it does say that the Constitution is designed to "form a more perfect union" than the Articles of Confederation. The Articles of Confederation explicitly stated that states were unable to secede.
The other point is that, if the Constitution is a contract between the states, legally you would need the consent of all parties to break off a contract. So any state that wanted to secede would have to receive the permission of all the other states.
Yet, please refer to our Declaration of Independence in which we seceded from British control.
NO!
The most prominent person in the North who believed that secession was unconstitutional was President Lincoln. He was in step with many other Northerners.As an aside, the Confederate constitution sought to clarify this issue for its own states. Confederate states were not legally allowed to secede form the Confederacy as a clause within their constitution forbid secession.
the practice of untouchability.
Religious Test
yes, true
As illegal and a violation of the US Constitution.
passing ex post facto laws
The oath of office and similar things? No, there is no such portion.
The original Constitution did forbid an official religion for the nation. However, everyone in the US has the right to religious freedom.
The Constitution provided no guidance on secession or readmission of states.
the answer is 10 years
It means that states can do what they want to do, IF the Constitution doesn't forbid it.
It forbid slavery and reconized equal rights.