Then is our preaching vain -- not simply as having some mixture of falsehood, but as being altogether an empty fallacy. For what remains if Christ has been swallowed up by death -- if he has become extinct -- if he has been overwhelmed by the curse of sin -- if, in fine, he has been overcome by Satan? In short, if that fundamental article is subverted, all that remains will be of no moment. For the same reason he adds, that their faith will be vain, for what solidity of faith will there be, where no hope of life is to be seen? But in the death of Christ, considered in itself, [1] there is seen nothing but ground of despair, for he cannot be the author of salvation to others, who has been altogether vanquished by death. Let us therefore bear in mind, that the entire gospel consists mainly in the death and resurrection of Christ, so that we must direct our chief attention to this, if we would desire, in a right and orderly manner, to make progress in the gospel -- nay more, if we would not remain barren and unfruitful. (2-Peter 1:8.)

Footnotes

1 - "C'est a dire, sans la resurrection;" -- "That is to say, apart from his resurrection."

And if Christ is not risen, then is our preaching vain - Another consequence which must follow if it be held that there was no resurrection, and consequently that Christ was not risen. it would be vain and useless to preach. The substance of their preaching was that Christ was raised up; and all their preaching was based on that. If that were not true, the whole system was false, and Christianity was an imposition. The word vain here seems to include the idea of useless, idle, false. It would be "false" to affirm that the Christian system was from heaven; it would be useless to proclaim such a system, since it could save no one.
And your faith is also vain - It is useless to believe. It can be of no advantage. If Christ was not raised, he was an impostor, since he repeatedly declared that he would rise Matthew 16:21; Matthew 18:22-23; Luke 9:22, and since the whole of his religion depended on that. The system could not be true unless Christ had been raised, as he said he would be; and to believe a false system could be of no use to any man. The argument here is one addressed to all their feelings, their hopes, and their belief. It is drawn from all their convictions that the system was true. Were they, could they be prepared to admit a doctrine which involved the consequence that all the evidences which they had that the apostles preached the truth were delusive, and that all the evidences of the truth of Christianity which had affected their minds and won their hearts were false and deceptive? If they were not prepared for this, then it followed that they should not abandon or doubt the doctrine of the resurrection of the dead.

Then is our preaching vain - Our whole doctrine is useless, nugatory and false.
And your faith is also vain - Your belief of a false doctrine must necessarily be to you unprofitable.

(5) And if Christ be not risen, then [is] our preaching vain, and your faith [is] also vain.
(5) The proof of that absurdity, by other absurdities: if Christ is not risen again, the preaching of the Gospel is in vain, and the credit that you gave to it is vain, and we are liars.

And if Christ be not risen,.... If this is a truth, and must be taken as granted, as it must be, if there is no resurrection at all:
then is our preaching vain; false, empty, delusory, unprofitable, and useless; not only that part of it which more especially concerns the resurrection of Christ, but even the whole of it; preaching Christ as the Son of God, which was the subject of the apostle's ministry, and which he set out with, is to no purpose, if he is not risen; for one considerable proof of his sonship depends upon his resurrection, which is the declaration of it; for who can believe him to be the Son of God, if he is detained under the power of the grave? one reason why he could not be held of death, and the pains and cords of it, any longer than was necessary, and was his pleasure, was because he was the Son of God, as well as surety of his people, who had paid the whole debt: so the preaching of his incarnation, obedience, sufferings, and death, is of no use and avail, if he has not abolished death, and brought life and immortality to light, first in himself, and then for his people:
and your faith is also vain; either the grace of faith, by which they believed on Christ, or the doctrine of faith; or since this is repeated, 1-Corinthians 15:17 the one may be meant here, and the other there. The doctrine of faith they had given their assent to, not only respecting the resurrection of Christ, but any other truth relating to his person and office, must be vain and empty, and without any foundation; even that faith which is one, uniform, harmonious, and consistent, which was once delivered to the saints; which they are to stand fast in, to strive, contend, and fight for, and not part with at any rate, upon any account whatever; and yet this, and the preaching and belief of it, are useless and insignificant things, if Christ is not risen; such wretched absurdities must follow upon the denial of that truth.

your faith . . . vain-- (1-Corinthians 15:11). The Greek for "vain" here is, empty, unreal: in 1-Corinthians 15:17, on the other hand, it is, without use, frustrated. The principal argument of the first preachers in support of Christianity was that God had raised Christ from the dead (Acts 1:22; Acts 2:32; Acts 4:10, Acts 4:33; Acts 13:37; Romans 1:4). If this fact were false, the faith built on it must be false too.

Then is our preaching - From a commission supposed to be given after the resurrection. Vain - Without any real foundation.