The 7 Thunders are the 7 Time Prophecies between 1798 & 1844 -31

There are those now living who, in studying the prophecies of Daniel and John, received great light from God as they passed over the ground where special prophecies were in process of fulfillment in their order. They bore the message of time to the people. The truth shone out clearly as the sun at noonday. Historical events, showing the direct fulfillment of prophecy, were set before the people, and the prophecy was seen to be a figurative delineation of events leading down to the close of this earth's history. The scenes connected with the working of the man of sin are the last features plainly revealed in this earth's history. The people now have a special message to give to the world, the third angel's message. Those who, in their experience, have passed over the ground, and acted a part in the proclamation of the first, second, and third angels' messages, are not so liable to be led into false paths as are those who have not had an experimental knowledge of the people of God... {2SM 102.1}

These relate to future events which will be disclosed in their order. Daniel shall stand in his lot at the end of the days. John sees the little book unsealed. Then Daniel's prophecies have their proper place in the first, second, and third angels' messages to be given to the world. The unsealing of the little book was the message in relation to time. {7BC 971.4}

The special light given to John which was expressed in the seven thunders was a delineation of events which would transpire under the first and second angels' messages. It was not best for the people to know these things, for their faith must necessarily be tested. In the order of God most wonderful and advanced truths would be proclaimed. The first and second angels' messages were to be proclaimed, but no further light was to be revealed before these messages had done their specific work. This is represented by the angel standing with one foot on the sea, proclaiming with a most solemn oath that time should be no longer. {7BC 971.6}