Thou shalt suck the milk of the Gentiles - This expression means, ‘Whatever is valuable and rich which they possess shall contribute to your welfare.’ The idea is the same substantially which occurs in the previous parts of the chapter, that the riches of the pagan world would become tributary 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 be made to contribute to sustain the church. See the sentiment explained in the notes at Isaiah 49:23.
And thou shalt know - By the protection which shall be extended to thee, and by the accession which shall be made to thee, thou shalt have full proof that Yahweh is thy protector and friend. The conversion of the pagan world shall demonstrate that Yahweh is the friend of his church and people.