Perhaps the one spiritual thing that is hard to validate from experience is what's beyond the grave.  No human being has died and come back to tell us what's there. (Jesus is the only exception but he's more that just human.)

Perhaps one way to go about answering this question is to follow a summary progression.  

#1 The Starting Point:  God exists.

#2 Point: God revealed himself and the future of the world in the Bible.

#3 Point:  The Old Testament and New Testament manuscripts are reliable and trustworthy on the matters of spirituality and life mentioned in their pages.

#4 Point:  The Bible says that Jesus claimed to be God.  He validated this premise through his life, his miracles and perhaps more than anything else, he beat death by rising from the dead.  Hundreds of people saw the risen Christ as documented not only in the Bible but also in many ancient extra-biblical sources.  So, because the Bible is reliable we can trust what Jesus said.

#5 Point:  Jesus spoke about the afterlife on numerous occasions.  He even mentioned that there is an actual place called heaven.  Everyone who is a follower of Christ will be in heaven with him when they pass from this life to the next life.      

Conclusion:  There is an afterlife.   

For more supporting reasons for the syllogistic progression, you may want to read Does God Exist? by J.P. Moreland.