The devil begins to manifest himself when he sees a small opening...we cannot give our enemy even a tiny entry without his immediately gaining the whole. We must not open the door to him lest he beat down all the walls, but as soon as he has a tiny crack he is inside. All the more, then, must we hold firm and doubly fortify ourselves with God's Word, knowing it constitutes our true weapons: it is our sword, our helmet,, our shield, our coat of mail (Eph. 6:17); in short, all the harness we will be equipped with to withstand all of Satan's calls to arms and repel them, provided we have the Word of God.. But we must be so practiced that the devil will find no opening, as I have just said, for just as soon as we begin to doubt or argue, or waiver, by has won the field, and we are conquered, we are lost people , unless God has pity on us. So let the Word of God serve us not only as a shield and sword and helmet and harness, but also as wall, as moat and rampart. And whenever the devil wishes to approach, let him find no way to do so, come what may. That is how we understand what John says, namely, that faith is our certain victory over the whole world, when we are so founded on God's Word that its power towards us will be complete that there will be no debating and that will be enough that God has spoken.

~John Calvin.