This practice of Paul corresponds to one of the earliest creedal statements of Christian faith:
The Apostles Creed: "I believe in the communion of saints."
Communion of saints refers to the bond of unity among all believers, both living and dead, who are or have been committed followers of Jesus Christ.
In the eyes of God, in eternity, the distinction between His People who are "living" or who are "dead" is not at all important.