Illumination is the act where God allows for the comprehension/understanding of God, God's will, or God's works. M.X. Seaman helpfully bifurcates the doctrine of illumination into two sub categories: Initial and Progressive. He terms the fusion of the two as transformative illumination. He defines it as such, “transformative illumination is that process whereby the Holy Spirit’s initial illumination enables his ministry of progressive illumination such that his work of transformation results in the life of the believer.”
In establishing a Trinitarian doctrine of illumination, it is necessary to appreciate the distinctions in the involvement of the different persons in the work of illuminating. The Father’s role in illumination is as the One who gives, drawing people to His Son.  The Father also allows for the blinding of mind. The Son’s role in illumination is as the instrument to creation; the light.  The Son also is the nexus between the Father and the Spirit’s illuminating work. The role of the Holy Spirit in illumination is of the agent that grants the understanding. Similar to the previously noted Trinitarian doctrine of inspiration,  the involvement of Trinitarian members in illumination may overlap and simultaneously feature distinctions.
 M. X. Seaman, Illumination and Interpretation: The Holy Spirit’s Role in Hermeneutics (Eugene, Oregon: Wipf & Stock. 2013), 4.
 Ibid., 157.
 Cf. 1 Cor. 2:9-15; John 6:44; Eph. 1:17; 2 Cor. 4:6. Mark A. McIntosh notes, “The Spirit of truth guides Jesus’s followers by conducting them farther and farther, beyond their present capacities, into the fullness of the Father’s life, which is given away to Jesus and through Jesus to the world, all by the power of this same Spirit.” in “Chapter 10: Trinitarian Perspectives on Christian Spirituality” in The Blackwell Companion to Christian Spirituality, ed. Arthur Holder (Malden, Mass.: Blackwell Publishing. 2005), 177.
 Cf. 2 Cor. 4:4. Note that it is sin that hardened the heart of the unbeliever, Heb. 3:12-13.
 Cf. John 1:1ff; 16:13-15; 2 Cor. 4:4; James Buchanan, The Office and Work of the Holy Spirit, 50.
 Cf. John 16:15.
 Cf. John 16:13-15; 1 Cor. 2:9-15; Eph. 1:17; James Buchanan, The Office and Work of the Holy Spirit, 50.
 William D. Barrick, "Inspiration and the Trinity" in The Masters Seminary Journal 24/2 (Fall 2013), 181.
 Cf. 2 Thess. 2:16-17; 2 John 1:3; Luke 24:45. This activity may be more focused during the incarnation, before the ascension.