To this we have two answers.
1. In the first place, if this illusion leaves permanent beneficial effects upon the character, gives victory over sin, fills the soul with love toward God and the purest philanthropy, destroys the fear of death, and adorns and beautifies the spirit with all excellences, it is infinitely better than any reality to be found on earth, and it should be earnestly coveted and diligently sought by every person.
2. But we may know that God manifests himself in Christian experience by the testimony of consciousness — the same testimony that assures us of the existence of the external world.
To demonstrate the existence of the material world, as we have shown, has been for ages "the puzzle of philosophers," as Tyndall styles it, many contending that the sphere of consciousness is limited to the operations of mind itself, and that it cannot directly cognize any thing external. The most that it can do it to infer that its sensations have an external, unknown, and forever unknowable cause. Those who deny the correctness of this inference deny the existence of matter, and resolve it into ideas. With idealists, the ego only exists; the mountain, river, and plain are only so many different modifications of the ego, or self.
At length Sir William Hamilton arose, and cut this metaphysical knot by boldly enlarging the sphere of consciousness to include the outer world.
So we reply that the soul illumined by the Holy Spirit is conscious, not only of its own subjective religious exercises, but of God, their external cause, impressing himself mysteriously upon the Spirit. In other words, we may have, when our perceptions are quickened by the Holy Spirit, the same knowledge of God as we have of the external world. Christians in advanced experience universally testify that they all know God.
— from Love Enthroned, Chapter 13.