The affirmation of extra-biblical revelation is foundational and distinctive to Quakerism. George Fox's central claim was that the light of Christ and the guidance of the Holy Spirit are immediately accessible to contemporary believers, not confined to biblical times or texts. The Inner Light is God's revelatory presence active today. This allows Quakers to claim that the Spirit continues to teach, guide, and reveal divine truth in the present.

Robert Barclay's Apology systematically defends this doctrine: the same Spirit that inspired scripture also guides the worshipping community and individuals seeking divine truth. This does not mean new doctrine supersedes biblical teaching, but rather that the Spirit illumines, applies, and develops biblical truth in new contexts. Both evangelical and liberal Quakers affirm this continuing revelation, though they may disagree about specific claims of Spirit guidance. This doctrine distinguishes Quakerism from traditions emphasizing 'sola scriptura' and allows for dynamic theological development.