John 1:9 is sometimes called 'the Quaker text,' because the early Quakers, based on the verse, believed that sufficient light was offered to every person" (David J. MacLeod. The Creation of the Universe by the Word: John 1:6-9. Bib Sac 160 July-Sept 2003: 305-320). Some Quakers refer to the “inner light” as an internal revelation if appropriately responded to can save. They use John 1:9 as the proof text: “[That] was the true Light, which lights every man that comes into the world.”
We know God reveals Himself generally to everyone through creation or nature as Paul discusses in Romans 1:18-20. We also know that God is disclosing Himself generally to everyone through the conscience planted in every person’s soul which Paul explains in Romans 2:14-15. Neither of these methods of revelation is salvific. Does God, however, additionally reveal Himself to every person through what Roman Catholic Cardinal Avery Dulles calls “revelation as inner experience” (Avery Dulles, Models of Revelation. Garden City, N.Y.: Doubleday, 1983, 76). Another Roman Catholic Cardinal, Karl Rahner, believes this inner, general revelation is universal and all that is necessary for salvation.
There are two different ways of interpreting John 1:9 even though both cannot be correct.
The first way John 1:9 is interpreted reads like this: Jesus is the True Light who enlightens every man who comes into the world since creation.
This view says that Jesus gives subjective illumination to every person. Some would say this revelation can save and others teach that this inner revelation is not salvific.
Roman Catholic Cardinal Karl Rahner teaches because God wants all people to go to heaven, He gives internal enlightenment to every person even atheists whom Rahner calls “anonymous Christians.” This of course contradicts what Jesus declared in John 14:6: “I am the way, the truth, the life, no man comes unto the Father but by me.” Michael Horton summarizes Rahner's view: "Faith does not come by hearing the Word of Christ (Ro 10:17) but by responding obediently to the natural light that one already has within" (Michael Horton. The Christian Faith. Grand Rapids: Zondervan, 2011, 145).
Quakers also hold a similar view referred to as the “inner light.”
The Quaker belief that the Inward Light shines on each person is based in part on a passage from the New Testament, namely John 1:9, which says, "That was the true light, which lighteth every man that cometh into the world." Early Friends took this verse as one of their mottos and often referred to themselves as "Children of the Light." Moreover, Friends emphasize the part of the verse that indicates that the Light "is extended to all people everywhere", even "people who have never heard of Christianity in a meaningful way or at all can share in the Light, if they sincerely respond to God's grace. For when Gentiles, who do not have the law, by nature do what the law requires, they are a law to themselves, even though they do not have the law. They show that the work of the law is written on their hearts, while their conscience also bears witness, and their conflicting thoughts accuse or excuse them on the day when, according to my gospel, God judges the secrets of men by Christ Jesus (Romans 2:14-16)" (“What Do Friends Mean by the Inward Light?" The Key to the Faith and Practice of the Religious Society of Friends. QuakersOnline. 2007. Retrieved July 26 2017.
Others, like John Calvin, in his Institutes wrote there is in every person a “natural instinct of some sense of deity.” But Calvin did not believe that this internal intuition was sufficient to save. Proof of this “natural instinct,” Calvin argued, is the knowledge of God in all people even primitive tribes who have never heard of the Bible or God. Calvin also connected this “natural instinct” to each person’s conscience. He wrote, the “worm of a guilty conscience torments them within, worse than if they were seared with hot irons” (John Calvin, Institutes. Grand Rapids: Eerdmans.1949, 1.3.1-3.) In his commentary, which was written after his Institutes, Calvin explains Jesus enlightening every man not only through each person's conscience but also with "the common light of nature" (John Calvin. Commentary on the Gospel According to John. Grand Rapids: Eerdmans, 1949, 38).
I think we could be safe in saying that Calvin believed in an internal witness which primarily operated through the conscience and also the external witness of nature but not as a means of salvation apart from the Gospel of Jesus Christ.
The second view is the preferred view and reads John 1:9 thus: Jesus is the True Light, who enlightens every man, who came into the world at His incarnation.
This view, which is D. A. Carson’s view (The Gospel According to John,123-124), advocates that since Jesus’ incarnation, He enlightens every sinner who gets saved. Wayne Grudem believes there is truth in both views: “Since the context of John 1 is talking about Christ coming into the world, it is better to take the phrase ‘was coming into the world’ to modify the true light, Christ, rather than every man, though both are grammatically possible. In either case, the verse still says that Christ enlightens every man” (Wayne Grudem. Systematic Theology. Grand Rapids: Zondervan, 1994, 659). Grudem believes that Christ enlightens every sinner with the ability to respond to general revelation in creation. This enlightenment, according to Grudem cannot save.
The context favors the second view. In verses 10-11, this Light came into the world at His incarnation and was rejected. But in verses 12-13, He can be received. Christ enlightens every sinner to salvation through the Holy Spirit as He described in John 16:7-11. The Holy Spirit with the gospel enlightens the satanically blinded eyes by convicting each sinner of rejecting Christ as Savior.
Gordon R. Lewis and Bruce A. Demarest hold loosely to both views: "The eternal Word created the universe (John 1:3), illumines human intellectual and moral faculties (John 1:4, 9), and, having become incarnate, renews spiritually those who believe in him (John 3:3, 16). Thus it seems clear that in John 1:4, 9 the apostle teaches that through the universal operation of the Logos the mind of every person is divinely illumined so as to perceive God as the inescapable datum of human existence. All people, by virtue of the general illumination of the Logos, reflect...awareness of the reality to God within their hearts or minds" (Gordon R. Lewis and Bruce A. Demarest. Intergrative Theology.Grand Rapids: Zondervan, 1996, 72). Instead of this inner general revelation of conscience or ability to rightly evaluate creation, Lewis and Demarest, see it as more of an intuition in every person that is not salvific. I do not see the internal intuition in John 1:9 nor in other texts.
Thomas Constable in agreement with the second view added: "The Light shines on everyone even though most people do not see it because they are spiritually blind. He shines even on those who have never heard of Him in that when He came He brought the revelation of God that is now available to everyone" (Lumina.Bible.Org).
The coming of Christ was like the rising sun. The rising sun shines on all people on half of the planet. Even for the people on the other side of the planet who are in the dark and do not see the light of the sun, the sun is still shining. The incarnate Christ is that Light. It is the believers’ responsibility to get people in the dark to that Light.
This is where we believers are part of this enlightening of the unconverted. Christ enlightens every sinner by means of the Holy Spirit who uses Christians who witness the saving gospel good news of Jesus death, burial, and resurrection. Jesus is the Light of the World and yet He commanded us, "Let your light so shine before men that they see your good works, and glorify your Father who is in heaven." For whom are you praying and becoming the answer to your prayer by the taking the gospel of Jesus Christ to them?