The momentous decree为主语，who had been seared in the flames of withering injustice是定语从句，来修饰millions of Negro slaves
整个句子其实就是
对millions of Negro slaves 来说，The momentous decree的到来，像a great beacon light of hope一样。
The momentous decree(as a great ......) -- came-- to millions of Negro slaves (who......)
句子中两处很长的定语：
1. as......hope 修饰 decree
2. who......injustice 以定语从句修饰slaves
再去掉形容词，整个句子的主干就为：
Decree came to slaves.
这么说行吗？
decree came as a hope to slaves who had been seared in injustice
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