What do people mean by being baptised on behalf of the dead? If the dead are not raised at all, why are people baptised on their behalf? Why am I in peril every hour?

St Francis de Sales comments:-

This passage properly understood evidently shows that it was the custom of the primitive Church to watch, pray, and fast for the souls of the departed. For, firstly, in the Scriptures ‘to be baptised’ is often taken for afflictions and penances; as in St Luke, chapter XII, where our Lord speaking of his Passion says, ‘I have a baptism wherewith I am to be baptised, and how am I straitened until it be accomplished!’ – and in St Mark, chapter X, he says, ‘Can you drink of the chalice that I drink of, or be baptised with the baptism wherewith I am baptised?’; in which places our Lord calls pains and afflictions baptism. This then is the sense of that Scripture: if the dead rise not again, what is the use of mortifying and afflicting oneself, of praying for and fasting for the dead? And indeed this sentence of St Paul resembles that of the Machabees, ‘It is superfluous and vain to pray for the dead if the dead rise not again’ .

But secondly, it must not be said that the baptism of which St Paul speaks is only a baptism of grief and tears, and not of fasts, prayers and other works. For thus understood, his conclusion would be very false. The conclusion he means to draw is that if the dead rise not again, and if the soul is mortal, in vain do we afflict ourselves for the dead. But, I pray you, should we not have more occasion to afflict ourselves by sadness for the death of friends if they rise no more? (‘The Catholic Controversy’, III, V).

Bellarmine adds:-

This interpretation fits best with what follows, ‘Why am I in peril every hour?’, as if to say, ‘why do some afflict themselves in praying for the dead, and why do I afflict myself in preaching the gospel, if there is no resurrection of the dead?’

And again:-

 It is objected that the apostle should not have said, ‘why are people baptised on their behalf?’ but ‘why are we baptised on their behalf?’, since all Christians pray for the dead. I answer that the apostle wished to argue not from the custom of Christians, which might be rejected by the unbelievers as being something new, but from the custom of the Jews, who like their ancestors and following the example of the Scripture fasted and prayed for the dead (‘Controversies’, VI, VI).

St Ephraim, ‘the harp of the Holy Ghost’, had already expounded St Paul in this way, sixteen hundred years ago.