Like many Isaiah passages these six verses can be difficult to understand. However, with patience and persistence, some sense can be made of Isaiah's meaning. The key to this passage is found in verse 5, I showed them for fear lest thou shouldst say—Mine idol hath done them. An interpretation of the rest of this passage is offered.
The phrase, I have declared the former things from the beginning, means that the Lord has told the children of Israel his doings (the former things) from the beginning of time down to Isaiah's time. His intent was to send a clear message of prophecy through his prophets so that when these prophecies came to pass, the children of Israel wouldn't give the credit to their idols. Long before Israel had turned away, the Lord knew that this would happen, I did it because I knew that thou art obstinate and thy neck an iron sinew, and thy brow brass…I knew that thou wouldst deal very treacherously, and was called a transgressor from the womb (v. 4,8). The phrase, Thou hast seen and heard all this; and will ye not declare them?, could be paraphrased as follows, "you have heard of my prophecies and seen their fulfillment from the beginning, won't you admit it? Or will you give the credit to your dumb idols?" The last half of verse 6 and verse 7 refer to new prophecies and their eventual fulfillment which will also be rejected by Israel because of their spiritual deafness (thou heardest them not) and hearts which knew not the Lord (thou didst not know them). The rebellion and idolatry of Israel is so bad that the Lord considers destroying them, but He defers his anger for His name's sake.