Posthumous baptism by proxy allows faithful Mormons to have their ancestors baptized into the 178-year-old church, which they believe reunites families in the afterlife.
Sorry Mormons, but what unites people in the afterlife is being saved by the covenant of grace and being united to Christ. Baptism is simply a sign and seal of the covenant which must be enterred through faith, not the posthumous works of others!
Using genealogy records, the church also baptizes people who have died from all over the world and from different religions. Mormons stand in as proxies for the person being baptized and immerse themselves in a baptismal pool.
Sorry again, but Jesus Christ is the only mediator who stands between God and man for salvation. And while water can cleanse the outward body, only the Spirit of God can cleanse the heart.
.... Quotes taken from here.