Eternal Decisions--Explain your answer

 There are three doors: 

one leads to eternal happiness,

one leads to eternal suffering, 

and one leads back to the starting point.

 Each door is guarded by a knowing 

but not entirely truthful being. 

The guard of the first door says,

 "The second door leads to eternal happiness."

The guard of the second door says, 

"The first door leads to eternal suffering."

 The guard of the third door says, 

"One of the other guards is always telling the truth, 

and the other is always lying." 

Knowing that only one of the guards is entirely truthful, 

which door should you choose?