Thy words were found, and I did eat them; and Thy word was unto me the joy and rejoicing of mine heart: for I am called by Thy name, O LORD God of hosts. – Jeremiah 15:16

Many professing Christians think very lightly of scriptural knowledge, and especially of an experimental acquaintance with divine truth. Few nowadays have studied the doctrines of grace so as to be able to give a reason for the hope that is in them. Too often converts are made by excitement, and, as a consequence, when the excitement is gone, they grow cold; some of them go back to the world, and prove that they were never taught of God, and others linger on in a half-starved condition, because soul-sustaining truth is hidden from them. The man who knows the truth, and feels that the truth has made him free, is the man who will continue a free man at all hazards. There are enemies of the faith about nowadays; error is put in very tempting forms. Those who try to subvert the gospel are exceedingly dextrous, and know how to make every falsehood fascinating. These will rend and devour, but who will be their victims? Not the instructed saints, not those who can say “Thy words were found, and I did eat them,” but the mixed multitude in nominal union with the church, who scarce know what they believe, or knowing it merely in the letter have no inward vital acquaintance therewith. We read in the word of God of certain deceivers who would, if it were possible, deceive the very elect, from which we gather that the elect cannot be deceived, and that for this reason-that the truth is not held in the hand of the elect man as a staff which can be wrenched from him, but he has eaten it: it has entered into his vital substance. You cannot tear away from a man what has become assimilated to himself. You might draw the silken thread out of a piece of tapestry, and in so doing injure the material, but you cannot remove the truth which is interwoven into the fabric of our new-born nature by the Holy Spirit. A Christian is dyed ingrain with the truth-he wears no flying nor fading colors; he can as soon cease to be as cease to believe what he has learned by the Spirit’s teaching. ~ C.H. Spurgeon

https://www.blueletterbible.org/Comm/spurgeon_charles/sermons/0980.cfm