When you agree with the ideas expressed by an author, it is ethical to use them without giving credit to the source?

No, it is unethical to use another author's ideas without crediting his or her work or literature. Failing to credit an author's work in your own work is known as plagiarism. Plagiarism has serious and sometimes even legal consequences in both educational institutions and beyond that in the corporate world and publishing industries. To avoid plagiarism and remain ethical it is best to quote ideas given by another author and clearly state the source from which you are quoting the author's work.