Thank you for your post.
First thing I would like to say is that praying for those who fell asleep in Christ was well established before Constantine ( You know what I mean).
Here is an interesting verse:
Otherwise, what will they do who are baptized for the dead, if the dead do not rise at all? Why then are they baptized for the dead?
We know that Paul is making his apologia for the resurrection in 1Cor 15. Paul cites what seems to be a practice of faith within the Corinthian church, in that members of the church would be baptized for those departed before receiving it.
While we know this is not practiced today, some observations:
1) Paul did not condemn this practice.
2) This shows that the Corinthians had a sacramental understanding of baptism.
3) That Paul acknowledged this was done for the dead.