Did early christians practice vicarious baptizing for the dead?
There are two opinions on this. One is that they baptized living people as a surrogate for a dead person. The other scenario is that they invoked the memory of departed loved ones during the baptismal ceremony. The latter case is more likely – there is a long-standing Christian tradition of commemorating the memory of the deceased during religious rites like baptism, the liturgy, vespers, etc. But there is no historical reference whatsoever to support the former view.
Yes, they did. Paul talks about it in Corinthians 15:29 “Else what shall they do which are baptized for the dead, if the dead rise not at all? why are they then baptized for the dead?” This verse has been changed by the writers of many modern versions of the Bible, so it is not found in all of the modern versions of the Bible. But is in the King James Version, which was translated in the 1600’s. Mormonism was established in 1830. No Mormon wrote that verse. Paul was making a point about the resurrection to the Sadducees, who did not believe in life after life. He was using as an example a well-known practice, to convince the Sadducees that there is a life after life.