To Exist
する verb
Know how 存在 is existence? Well, make it a verb and then you have to exist.
そんざいする
This is a jukugo word, which usually means on'yomi readings from the kanji. If you know the readings of your kanji you'll know how to read this as well.
私は死後の世界は存在すると思います。
I think that life after death exists.