Errors of accretion occur when churches add their own idiosyncratic doctrines to the unchanging core of essential Christian truths as if they, too, must be believed to be saved. Today these might include a particular Protestant theological system (dispensational or covenant), a certain form of church government (episcopal, presbyterian, or congregational), a dogmatic view of the atonement (demanding that people not only believe that Christ’s death and resurrection save us, but being able to explain exactly how it saves us), or a certain hermeneutic (historical-grammatical, theological, canonical, or Christocentric). By looking back, we can be constantly reminded that the core doctrines of the faith that mark us as true Christians—things like the Trinity, the person and work of Christ, salvation by grace, the authority of Scripture—cannot be added to without obscuring the Christian faith. Distinctive doctrines can be held by different denominations within the bounds of orthodoxy, but those distinctions and different emphases should never be held up as marks of orthodoxy.
The only caveat I might add is to the first sentence “Errors of accretion occur when churches add their own idiosyncratic doctrines to the unchanging core of essential Christian truths as if they, too, must be believed to be saved [or sanctified].” Legalism cuts across our faith and does not just affect salvation. If it first does not encroach on the core of what it means to be Christian, then it rears its ugly head getting in the way of progress toward our maturity. Caveat Emptor!