I have to say that it is very hard to ever define a literal meaning of a poem unless the poem includes footnotes the author alone has included regarding the meaning. All poetry is subjective- the reader alone can define meaning behind a poem. That being said, there are typically multiple meanings behind every poem. What I can offer you is my interpretation of what the poem means with the hopes that it will help you to find your own meaning.
What I will do is dissect the poem line by line so as to decipher the meaning of the poem as a whole.
"Cruelty has a human heart"- The heart, here, is personified. Cruelty is given the ability to have functions that only a human can possess. Cruelty is the pleasure one has in causing pain to others. Many times, those we love hurt us.
"And Jealousy a human face"- Blake personifies "jealousy" by capitalizing it. Again, this abstract idea, jealousy, is transformed to something concrete so that it can be solidified as.
"Terror the human form divine"- Personified once again (an assumption given terrified occurs at the start of the line and needs to be capitalized, but assumed since the other abstract ideas have been personified). Blake is, like in the lines before, concreting the idea of terror for the reader.
"And Secrecy the human dress"- When a person keeps a secret, they must change the truth about what they are hiding- basically, they are maskingthe truth. Dress is something we do to hide our bodies from the world (can be tied to the Garden of Eden).
"The human dress is forged in iron"- This references back to the last line of the previous stanza. Blake is stating that secrets are made of iron. Nothing will break the strength, or should break the strength of a secret. They should not be made or kept lightly.
"The human form a fiery forge"- This references back to the third line of the previous stanza. Fiery forge describes something so hot it can melt metal. Something as hot as a forge can bring terror to the mind. It could also be referring to terror as being something that can change a person from who they are to something very different.
"The human face a furnace sealed."- This, again, references two different ideas. The human face, much like molten metals removed from a furnace and left to harden, can become something very different than what one wishes it to be. Jealousy can can the face of a person; once the furnace door is closed, there is no returning to change what something has become. It will be like that until the furnace is reopened.
"The human heart is a hungry gorge"- Human hearts are boundless to the extent to which one can fill it. When a person is hurt or angry, our hearts typically become so filled with hated or rage that it never seems to end. To be honestly cruel takes a person who has no void to the anger and hatred their heart can hold.
Basically, Blake is personifying emotions that people feel, and then redefining (creating a metaphor) the personified body part to a concrete, but not human object. What Blake is creating is a circular pattern which shows the bitter and cruel path one can fall into when one becomes cruel or jealous, and/or has feelings of terror, and/or what carrying a secret can/will do.