Albert Barnes Commentary Isaiah 60:16

"Thou shalt also suck the milk of the nations, and shalt suck the breast of kings; and thou shalt know that I, Jehovah, am thy Saviour, and thy Redeemer, the Mighty One of Jacob." — Isaiah 60:16 (ASV)

Thou shalt suck the milk of the Gentiles - This expression means, ‘Whatever is valuable and rich which they possess will contribute to your welfare.’ The idea is substantially the same as that which occurs in the previous parts of the chapter: that the riches of the pagan world would contribute to the advancement of the true religion.

And thou shall suck the breast of kings - The Chaldee renders this, ‘And thou shalt be satisfied with the riches of the people, and shalt delight thyself with the spoil of kings.’ The phrase ‘to suck the breast of kings’ is unusual, but the sense is simple and plain: that kings and their wealth should contribute to sustain the church. See the sentiment explained in the notes on Isaiah 49:23.

And thou shalt know - By the protection which will be extended to you, and by the accession which will be made to you, you will have full proof that Yahweh is your protector and friend. The conversion of the pagan world will demonstrate that Yahweh is the friend of his church and people.