James 2:19

• Thou believest that there is one God; thou doest well: the devils also believe, and tremble. (James 2:19)

Some will argue that this proves that believing the gospel isn't enough.  But is believing there is one God the whole gospel?

To review what's declared as the gospel:

• ¶ Moreover, brethren, I declare unto you the gospel which I preached unto you, which also ye have received, and wherein ye stand; By which also ye are saved, if ye keep in memory what I preached unto you, unless ye have believed in vain. For I delivered unto you first of all that which I also received, how that Christ died for our sins according to the scriptures; And that he was buried, and that he rose again the third day according to the scriptures: And that he was seen of Cephas, then of the twelve: After that, he was seen of above five hundred brethren at once; of whom the greater part remain unto this present, but some are fallen asleep. After that, he was seen of James; then of all the apostles. And last of all he was seen of me also, as of one born out of due time. (1 Corinthians 15:1-8)

We can see that believing in one God isn't the whole gospel.  In the context of other scriptures, it's part of the gospel, but not the whole thing.

Another point that can be made is that the gospel isn't for devils.  It's for people, as the context makes clear:

• For as in Adam all die, even so in Christ shall all be made alive. But every man in his own order: Christ the firstfruits; afterward they that are Christ's at his coming. (1 Corinthians 15:22-23)

Did devils come from Adam?  No, the gospel is for humankind:

• But we see Jesus, who was made a little lower than the angels for the suffering of death, crowned with glory and honour; that he by the grace of God should taste death for every man. (Hebrews 2:9)

So to use this verse to bolster support for the claim that believing isn't enough is simply an unfounded argument.  It's what you believe to be saved that matters, and that you're a person with the possibility of redemption, which the devils don't have as their judgement is already set:

• ¶ And when he was come to the other side into the country of the Gergesenes, there met him two possessed with devils, coming out of the tombs, exceeding fierce, so that no man might pass by that way. And, behold, they cried out, saying, What have we to do with thee, Jesus, thou Son of God? art thou come hither to torment us before the time? (Matthew 8:28-29)