I have not spoken in secret: God's promises are clear, though they do not always specify how or when they will happen.
And now the Lord GOD has sent me, accompanied by His Spirit: The identity of the one sent is unclear. Isaiah might be referring to himself as a prophet inspired by the Spirit, speaking God's words to Israel. The connection with the spirit suggests it refers to the promised servant, the Messiah.
The presence of the Spirit ensures the servant's success.