When people eventually got bored of Jesus's magic act where he saws himself in half, and his mutant power to make unlimited alcohol from the water he's standing on, he decided to wow them once and for all by building a time machine. This would also let him become his own dad by banging his mom. So he built and christened it the Tarkus, after his favorite album. He than preceded to settle the great Summa Theologica debate, between Thomas Aquinas, Einstein and Socrates, about whether or not he could break the philosophers legs with a gold club from the future.
Design and concept
The TARDIS (full name RETARDIS) or TURDIS as it is now called is essentially a self-propelled time machine that utilities the scientific principles of its main source of power, that is, a pile of crap. Inside the machine is said to be bigger than the outside, but this is probably a hallucination caused by inhaling of noxious fumes emanating from the endless cesspit below. Certainly all incarnations of the Doctor who have used it previously have all been either crazy or exhibited signs of chemical poisoning.