Ephesians 6:12-13 (NIV)
12 For our struggle is not against flesh and blood, but against the rulers, against the authorities, against the powers of this dark world and against the spiritual forces of evil in the heavenly realms. 13 Therefore put on the full armor of God, so that when the day of evil comes, you may be able to stand your ground, and after you have done everything, to stand.
Those who would approve themselves to have true grace must aim at all grace, the whole armour. It is called the armour of God, because he both prepares and bestows it. We have no armour of our own that will be armour of proof in a trying time. Nothing will stand us in stead but the armour of God. This armour is prepared for us, but we must put it on; that is, we must pray for grace, we must use the grace given us, and draw it out into act and exercise as there is occasion. The reason assigned why the Christian should be completely armed is that he may be able to stand against the wiles of the devil-that he may be able to hold out, and to overcome, notwithstanding all the devil's assaults, both of force and fraud, all the deceits he puts upon us, all the snares he lays for us, and all his machinations against us. [Matthew Henry Commentary]
As [one researcher has] described it, our brain blinds our mind to the unusual. For instance, in one study, researchers put a clown on a unicycle in the path of pedestrians. The researchers asked people who walked passed the clown if they had noticed anything unusual. Everybody saw him unless they had been on their cell phone. Three out of every four people who had been using their phone did not see the clown. They looked back in astonishment, unable to believe they had missed him. They had looked straight at him but had not registered his presence. The unicycling clown crossed their paths but not their minds. [Kevin Ashton, How to Fly a Horse (Doubleday, 2015), page 97]
Life is complicated by all the things that come at us that we do not see coming. This is why it is important to put on the armor of God so we can protect ourselves from those things we do not see coming.