On the polemical creationist side, the sin is intellectual dishonesty. It begins innocently as a wise recognition that faith must precede reason, even if the faith is only in reason itself (as Gödel showed, reason cannot prove its own validity). But under pressure from a contemptuous academic elite the appeal to faith rapidly becomes anti-intellectualism and what Socrates identified as a great sin, "misologic" or treason against the Logos, against reason itself -- in religious terms, a sin against the Holy Spirit. Under further pressure it resorts to rhetorical dishonesty and hypocrisy, to an attempt to appropriate the garments of science and reason, and so we get "creation science", the misuse of the term "intelligent design", the whole grotesque solemn sham of pseudoscientific periodicals and conferences on creation science, and a lame parade of scientific titles and degrees. A lie repeated often enough convinces the liar, and many creationists may now have forgotten that they are lying at all.
Update: Meanwhile, some Catholic scientists are asking the new Pope for clarification.
Whenever creationism pops up among Catholics, I think of Saint Augustine's warning (in his commentary on Genesis--I had mistakenly thought it was City of God). Christians can't afford to be stupid about science. It only exposes the faith to ridicule.
Like we need that....