Laying something else down alongside the Gospel is to turn away from the gospel altogether. This was a SURPRISE to the apostle--which warns us that we must be watchful of ourselves, for this happens to those about whom it is a surprise. The SERIOUSNESS of this is not just because an important doctrine is at stake, but because turning away from the gospel is turning away from a Person--Jesus Himself. And the SEVERITY of the consequences are such that those who turn away from Jesus suffer everlasting condemnation under His wrath.