Chapter 7 tells us that there will be a worldwide revival take place during the Tribulation. It will not occur within the Church Age but during the Tribulation Period. This is another calm before the storm. They were given power to hurt the earth and sea. The fifth angel s instruction to the first four angels was to not hurt the earth, sea or trees until the servants of God receive the seal of God in their foreheads.
Seal is a mark of possession, authority, power, protection and preservation.
And the LORD said unto him, Go through the midst of the city, through the midst of Jerusalem, and set a mark upon the foreheads of the men that sigh and that cry for all the abominations that be done in the midst thereof. Then they began at the ancient men which were before the house. This event reminds us of the plagues in Egypt that the children of Israel were kept from. The most devastating was the last one of every first born son being killed by the Death Angel. The people of God were protected because they had applied the blood of the lamb.