"In illumination, the Spirit both enlightens the mind, making it capable of receiving spiritual things, and impresses on the mind the objective reality of those things of which the word of God bears witness. The knowledge of spiritual realities thus given is as immediate and direct in its way as is the knowledge of material things which we gain by sense, and it brings with it a quality of immediate conviction analogous to that which sense perception brings."
-J.I. Packer, A Quest for Godliness

Illumination- “A true sense of the divine excellency of the things revealed in the Word of God, and a conviction of the truth and reality of them.”
-Jonathan Edwards, “A Divine and Supernatural Light”

“He gives unto believers a spiritual sense of the power and reality of the
things believed, whereby their faith is greatly established…the divine
witness…is the greatest corroborating testimony whereof we are
capable…And when believers have attained hereunto, they do find the
divine wisdom, goodness, and authority of God so present unto them as
that they need neither argument, nor motive, nor anything else, to
persuade them unto or confirm them in believing.”
-John Owen, A Discourse Concerning The Holy Spirit