We are blinded when we are separated from God. What separates us from God? Sin. Since we are all born having a sinful nature, immediately upon entering this world we inherit spiritual blindness. This sinful nature is ingrained into our mortal flesh, making it weak and vulnerable to sinful urges and thoughts, all of which are inspired by Satan.
If we are aware of this truth, we can also learn this: Not everything that feels or seems good to us is from God. In fact, some things in our lives that feel good are actually sinful and self-destructive in nature. If this is indeed true, how can we ever completely trust ourselves to know what is right or wrong? The answer is, we can't; nor should we try. God tells us we must trust Him with all of our heart and not lean on our own understanding to guide us through life.
The perfect scenario for us in this life is to only be subjected to God's truth. However, we live in a sinful world; we are surrounded by lies and destructive, sinful pleasures. Quite simply, as long as we are alive on this earth, there is no way to completely avoid being influenced by the evil all around us. For example, even if you don't smoke cigarettes, if you stay in a smoke filled room long enough you'll eventually leave smelling like smoke.
Most if not all of the pleasures found in this world feed our physical needs and desires. In some cases, there is no way to avoid doing this. However, it is important that we become disciplined when indulging or satisfying these needs.
I believe our spiritual diet parallels our physical diet: Those things in this life which cater to our flesh are like junk food. Junk food tastes really good...but when we eat too much, it will often make us sick and give us an upset stomach. Also, if we only eat junk food and never give our bodies the healthy food that it desperately needs, our health will deteriorate over time. The same is true in our spiritual life: If we only feed the desires of our physical body but ignore the desires of our spiritual body, we will become spiritually sick and unhealthy.
The level by which we eliminate sin from our lives and resist the urges of our flesh, will have a direct effect on our level of spiritual blindness. The more we feed our spiritual body with God's Word and Truth, the more our blindness will give way to spiritual clarity and wisdom. God's truth will become like a light inside our hearts, guiding and leading us down every good path. Once we have received this light inside of us, we will no longer stumble through life in spiritual darkness.
We must learn to discern right from wrong in all matters, and to test the spirits which try to influence us on a daily basis. Think of our spiritual battle like this- The devil sits on one of our shoulders with a megaphone, screaming lies and temptations into our ear. On the other shoulder rests the Holy Spirit, whispering very softly and gently things from God.
Because the devil's voice and influence is much louder in this world, it can seem nearly impossible to hear God's voice. Therefore, we must learn to muzzle the devil's voice. How do we do this? By resisting the empty, sinful pleasures found in this world, and embracing the hidden treasures found only in God. When we sow in the flesh, we reap emptiness, sin and vanity. But when we sow in the spirit, we reap the fruits of the Spirit of God. Doing this will also help remove the spiritual blindness that is present in our lives.
(Scripture references: Proverbs 3:6, Psalm 119:97-105, Isaiah 59:9-10, 1 John 4:1-6, Galatians 6:8, Luke 11:34-36)