Let me share a short explanation about this RC practice of praying for the dead.
According to RC teaching, Salvation is by Faith, but not by Faith alone. Faith alone is not sufficient to obtain Salvation. To be saved one must also observe the RC church commandments and the RC Sacraments i.e. the Sacrament of Baptism and the Sacrament of Penance (Confession to the Priest, absolution of sin by the priest, and performance of acts that gain Indulgences).
What are Indulgences?
When sins are committed they must be confessed to the priest, who then grants absolution. However, absolution only remits eternal punishment, the sinner is save from eternal hell but not from temporary punishment which the sinner must suffer here on earth or in the purgatory.
- An Indulgence is the partial or full remission
of the temporal punishment due to sin. In contrast
to partial Indulgence, a plenary Indulgence which
can only be granted by the pope, remits all of
the temporal punishment.
Indulgences can be only applied to oneself or to the dead in the purgatory, it can not be applied to any other living human beings. Prayer for the dead can gain an indulgence to reduce one's stay in the purgatory.
What the bible says on this matter?
Hebrews 10:11-18
New King James Version (NKJV)
Christ’s Death Perfects the Sanctified
11 And every priest stands ministering daily and offering repeatedly the same sacrifices, which can never take away sins. 12 But this Man, after He had offered one sacrifice for sins forever, sat down at the right hand of God, 13 from that time waiting till His enemies are made His footstool. 14 For by one offering He has perfected forever those who are being sanctified.
15 But the Holy Spirit also witnesses to us; for after He had said before,
16 “This is the covenant that I will make with them after those days, says the Lord: I will put My laws into their hearts, and in their minds I will write them,”[a] 17 then He adds, “Their sins and their lawless deeds I will remember no more.”[b] 18 Now where there is remission of these, there is no longer an offering for sin.