How Does Darkness Enter a Man's Heart?
Unlike what most people think, darkness does not enter a man's heart when he sins. It actually enters the moment he fails to give God the glory and a rightful place in his heart. This opens him up to all kinds of evil thoughts and deeds.
It is clearly written in the Romans 1:21:
For although they knew God, they neither glorified Him as God nor gave thanks to Him, but their thinking became futile, and their foolish hearts were darkened. Romans 1:21
Once darkness has entered a person's heart, all sorts of sins and corruption follow, and the man will become increasingly separated from God.
And even as they did not like to retain God in their knowledge, God gave them over to a reprobate mind, to do those things which are not convenient; Being filled with all unrighteousness, fornication, wickedness, covetousness, maliciousness; full of envy, murder, debate, deceit, malignity; whisperers ..... Romans 1:28-29
When Did Darkness Enter These People?
Adam and Eve
This happened to Eve when she decided to believe the serpent rather than God. Then it happened again to Adam when he decided to follow Eve's advice instead of the Lord's commandment. When they failed to give the Lord His rightful place, darkness entered their hearts and their descendants become more and more corrupted.
Saul
The Lord anointed Saul as King of Israel, but he followed his own heart instead of obeying the Lord's commandment to destroy all Amelakites completely. The Spirit of God then left him, and darkness entered his heart. He made more and more mistakes until he was finally replaced by David.
Solomon
Solomon inherited a strong Kingdom from his father David, and was given great wisdom by the Lord Himself. However, he later took many wives and worshipped their idols instead of the God of Israel. When he failed to give God the glory and be thankful, darkness entered his heart. Internal conflicts arose, and his Kingdom soon fell apart.
How to Protect Ourselves Against Darkness?
The Bible teaches us to guard our hearts diligently. If we always give glory to God, be thankful to His provisions, and obey His commandments, darkness will never have a chance to enter.
Guard your heart with all diligence, for from it flow springs of life. Proverbs 4:23