A number of evangelical Christian denominations and networks uphold a specific doctrine of Scripture, stating that the Bible is the ‘inerrant’ word of God. Those who adhere to biblical inerrancy tend to reject literary interpretations of the creation accounts in the Bible and therefore to reject evolutionary theory. Indeed, evolution rejection frequently functions as a key boundary for biblical inerrantists that must be strictly maintained. In this comparative study, we analyse interview data and other materials to uncover the mechanisms by which evolution rejection as a boundary is strengthened, maintained or weakened within two evangelical church congregations that adhere to biblical inerrancy: one in London, UK, the other in Texas, US. We find significant differences in boundary work between the two congregations and consider how the interplay of three factors—1) orientation of the congregation (internal or external), 2) religious context (minority or majority), 3) boundary salience—may lead to boundary strengthening or weakening.