I have declared, and have saved, and I have shewed, when there was no strange god among you: therefore ye are my witnesses, saith the LORD, that I am God.
I have declared - predicted the future (Isaiah 41:22-23).
And have saved - the nation, in past times of danger.
And I have showed - namely, that I was God.
When (there was) no strange (god) among you - to whom the predictions uttered by me could be assigned. "Strange" means foreign-introduced from abroad.