“If ... thine eye be single, thy whole body shall be full of light.”—Matthew 6:22.

Singleness of purpose, wholehearted devotion to God, is the condition pointed out by the Saviour's words. Let the purpose be sincere and unwavering to discern the truth and to obey it at whatever cost, and you will receive divine enlightenment. Real piety begins when all compromise with sin is at an end. Then the language of the heart will be that of the apostle Paul: “This one thing I do, forgetting those things which are behind, and reaching forth unto those things which are before, I press toward the mark for the prize of the high calling of God in Christ Jesus.” “I count all things but loss for the excellency of the knowledge of Christ Jesus my Lord: for whom I have suffered the loss of all things, and do count them but dung, that I may win Christ.” Philippians 3:13, 14, 8.MB 91.2

But when the eye is blinded by the love of self, there is only darkness. “If thine eye be evil, thy whole body shall be full of darkness.” It was this fearful darkness that wrapped the Jews in stubborn unbelief, making it impossible for them to appreciate the character and mission of Him who came to save them from their sins.MB 91.3

Yielding to temptation begins in permitting the mind to waver, to be inconstant in your trust in God. If we do not choose to give ourselves fully to God then we are in darkness. When we make any reserve we are leaving open a door through which Satan can enter to lead us astray by his temptations. He knows that if he can obscure our vision, so that the eye of faith cannot see God, there will be no barrier against sin.MB 92.1

The prevalence of a sinful desire shows the delusion of the soul. Every indulgence of that desire strengthens the soul's aversion to God. In following the path of Satan's choosing, we are encompassed by the shadows of evil, and every step leads into deeper darkness and increases the blindness of the heart.MB 92.2

The same law obtains in the spiritual as in the natural world. He who abides in darkness will at last lose the power of vision. He is shut in by a deeper than midnight blackness; and to him the brightest noontide can bring no light. He “walketh in darkness, and knoweth not whither he goeth, because that darkness hath blinded his eyes.” 1 John 2:11. Through persistently cherishing evil, willfully disregarding the pleadings of divine love, the sinner loses the love for good, the desire for God, the very capacity to receive the light of heaven. The invitation of mercy is still full of love, the light is shining as brightly as when it first dawned upon his soul; but the voice falls on deaf ears, the light on blinded eyes.MB 92.3

No soul is ever finally deserted of God, given up to his own ways, so long as there is any hope of his salvation. “Man turns from God, not God from him.” Our heavenly Father follows us with appeals and warnings and assurances of compassion, until further opportunities and privileges would be wholly in vain. The responsibility rests with the sinner. By resisting the Spirit of God today, he prepares the way for a second resistance of light when it comes with mightier power. Thus he passes on from one stage of resistance to another, until at last the light will fail to impress, and he will cease to respond in any measure to the Spirit of God. Then even “the light that is in thee” has become darkness. The very truth we do know has become so perverted as to increase the blindness of the soul.MB 93.1