Muslims, Jews and Christians all believe in a petulant insecure deity that casually practices mass murder on a gargantuan scale…

To pretend that one religion has cornered the market on silly is bigotry- and leads to atrocities such as those in the video you linked to…

https://www.youtube.com/watch?v=KPXpINM0BbM

Mormon theology takes seriously the scriptures that state that all people need to be baptized and receive other saving ordinances. Mormons aren’t the only religion that takes this scripture seriously, but the theology is unique to them in that it provides a method and an attempt to accomplish it.

The often misunderstood Mormon practice of baptism for the dead is that method. Mormons search out their own ancestry in order to provide the saving ordinances for them by performing proxy ordinances. Its been a part of Mormonism since the start and one of the main reasons we build temples, which is where the proxy ordinances are administered.

The Catholics and some other religions try to bridge the gap between scripture and practice by baptizing infants. Mormon theology finds infant baptism lacking in that it is limited to the living and there is a lack of choice on the part of the infant. Mormon theology and specifically Book of Mormon scripture condemn the practice of infant baptism and require a child reach an age of accountability elsewhere defined as 8 years old. Any child that dies before 8 years old or is mentally deficient is judged unaccountable and saved in Christ without requiring baptism. As far as choice goes, the ordinances performed on behalf of the dead can either be accepted or rejected by the dead in the hereafter. Mormons perform the ordinances in faith, but make no judgement as to whether it was accepted or not.

There are admittedly some problems with the theology and practice. The Church has a policy in place that one is only supposed to perform the ordinances for families in ones direct lineage and limits that work as well by requiring permission if there are living relatives that are closely related. Also holocaust victims are strictly forbidden to have the ordinances performed. I believe the policy was put in place to appease Isreal where the church operates a university campus. It isn’t well policed though and every so often there is a dust up about unauthorized ordinances being performed, followed by apologies.

The scarcity of records beyond 500 years ago is also a problem without a solution outside of more theology about records being revealed miraculously in the future to accomplish this. Generally considered to be a nonissue among Mormons who have enough on their plate with the records we have.

Convincing or not, the practice is a very central tenant of Mormonism as evidenced by the vast amounts of data collected and organized by the church for this purpose as well as the resources spent in building the temples to accomplish it.

I’m sure this was more than you expected and I tried to keep it as succint as possible, but geneology and baptism for the dead is so closely linked in the theology that you can’t explain one without the other.

My own ancestry back several hundred years has been meticulously recorded. Its one benefit of growing up Mormon. Further than 1500 it gets very murky. The scandinavian patronymics dead end after a while.