For sake of discussion...if we were under the OT, which this is, you may have an argument presuming Maccabees is an accepted part of the OT canon.
However, we're under the NT. The only acceptable sacrifice for sin is the death of Christ on the Cross.
The Pope was making a promise not based on the NT.
Hence, he was introducing a false promise to the soldiers.
There is no allowance in the NT for praying for the dead or offering a sacrifice on their behalf.
ealgeone:
Again, the OT prefigures the NT, so no, we are not under the OT, however, the NT is the fullfillment of the OT.
Praying for the dead would not be inconsistent with NT. We are bound by time, not God. Can God in ways only known to Himself make effective the prayers of the Church for the dead? In Catholic Theology, the answer is Yes. As To How, that is a Holy Mystery.
And effectiveness of those Prayers for the dead in the Church can only be effective because of Christ Death and Resurrection, apart from that, it is nothing.
Without getting into a long discussion on this, there are numerous examples from the ECF that mention the Church in Her Liturgy praying for the dead. I know you would not totally accept those ECF writings since you come from a sola scriptura perspective, however, it does indicate what was a common practice in the Church well before the NT canon was finally settled in the late 4th century.