Jacob not only lies to his father, but in Genesis 27:18-20, he invokes God to formulate his lie. To believers, lying when God is involved is much worse than simply lying alone—you break two commandments at once doing that: the ninth commandment prohibiting lying, and the third commandment prohibiting the misuse of God's name (commandment numbers vary by denomination). This doubly-wammy is often why, when people are suspicious, you'll hear them demand that the person "swear to God" that they're telling the truth. That way, if they're lying, they won't just go to the place in Hell reserved for liars, they'll go to the especially fiery and brimstoney section of Hell.
Of course, asking someone to "swear to God" for the purpose of exposing a lie is, in itself, breaking of the third commandment. Such an act is also strictly forbidden according to Matthew 5:33-35, which says that swearing on anything in the heavens or on earth is evil. Of course, I'd wager that most people who swear to God probably haven't read the bible anyway.
Coincidentally enough, Jacob's response is incredibly typical to the response that believers give when they reach the extent of their scientific knowledge. If you ask a believer where the universe came from, and they have no knowledge of the cosmos, you'll probably get Goddidit right off the bat. However, if you ask a believer who studied cosmology, they would probably tell you that the universe began during a period of inflation, they'd probably mention redshifted galaxies, the cosmic microwave background, abundance of primordial elements, etc. But finally, when asked what caused the big bang, they would cheerfully interject, Goddidit!, rather than say "I don't know."