This is said by Antonio in Shakespeare's Merchant of Venice.
This means that the devil is a [fallen] son of God he can cite, or quote scripture as well as any Biblical scholar, and twist the words to his benefit according to Christian beliefs.
He starts the play sad, and seems to have a melancholy air about him during the trial scene. Some people think he has an unrequited homosexual passion for Bassanio. You can take that or leave it. He is also an Anti-Semite and rude to Shylock and all the other Jews; we see this when he calls Shylock a "devil" ("The devil can cite Scripture for his purpose."). He is perhaps not quite as nasty as some of the other Venetian anti-Semites, but is bad enough that Shylock fixes his revenge on him.
He allowed and promoted the spontaneous demonstrations and processions because he is a dick.
Plagiarism can still occur even if you cite sources, if you don't properly attribute the ideas or words you use.
Purpose
As long as you cite your source, it is not considered plagiarism. But most teachers do not allow you to copy a page, even if you cite it.
Yes. Citation in a paper is a matter of academic honesty, not ownership. You would cite references even to other papers that you had written.
Yes, even when you paraphrase information from a source, you should still cite it to give credit to the original author and avoid plagiarism.
to separate metal from ore for the purpose of electroplating separation of useful metals using electochemistry
What is Cite
Yes, it is important to cite the source when summarizing information to give credit to the original author and avoid plagiarism. Even if you are putting the information in your own words, it is still necessary to acknowledge where the information came from.
Yes, when conducting research, you need to cite a source even if you have the keyword. Citing sources is important to give credit to the original author and to provide evidence for your claims.