Do all religions lead to God, or is there only one true faith? And if there's only one, then which one is it?
No "Religions" lead to God...
Only Jesus can lead to God (Christianity's not a religion...it's a relationship with God through Jesus Christ).
I'm not being arrogant with this answer, just answering your question in all honesty. I tried finding God in "other religions" (or religions in general), and never found Him. It was only after I finally "gave Jesus a chance", that I finally found and was able to get to know God.
Another way of looking at this is a story that I'd heard on the radio the other day (sorry, I'll probably kill this):
The preacher said that he was once in search of a sinless savior, one who could forgive him of his sins.
- He first went to the Buddhists and asked if they have a sinless savior. They replied that Buddhism does not have a sinless savior, but that they could help him obtain harmony.
- So the preacher went and asked a Hinduist. The hinduist replied that their religion does not have a sinless savior, but enlightenment.
- So the Preacher went to the Muslims to ask, although already knowing that Muhammad had spoken of needing his sins to be forgiven for, but he decided to ask anyway. But the Muslims replied that they do not have a sinless savior, but a prophet.
Only in Christianity will you find a sinless savior who can and will forgive you of your sins, if you ask Him to.
Jesus said in John 14:6, "I am the Way, the Truth, and the Life. Nobody comes to the Father (God) but through me." Only in Jesus is there the forgiveness of sins, and only through Jesus are we able to get to God the Father. There is no other way.