Thank you for the interpretative question, “In James 2:23, does the use of the word “fulfilled” (ἐπληρώθη) suggest that Genesis 15:6 was a prophecy about the future?”
As I read Genesis15 and James 2 again, the answer is yes in that Genesis 15:6 is prophetic since it was a promise made by God to Abram.
Genesis 15 states that Abram questioned the LORD God about his childlessness and who will inherit the promise given to him. His living relative was Eliezer of Damascus It is interesting to note that he doesn’t even mention his nephew, Lot. God takes Abram outside of his tent to look at the stars and makes him a promise that his descendants will be like the stars. It is astonishing to think that Abram believed God and at that moment in time, God saw into his heart and mind, his faith in God’s words to him. That action of faith by Abram was credited or placed in his account as trusting God. Abram already believed in God as he left Ur of Chaldea and that faith is further developed in his belief on heirs. For Abram, it was a forward looking faith of what is to come even though he doesn’t possess it in the present. That child wouldn’t come for years and he still continues to hold onto God’s promise when he thought that Ishmael would be the heir through Hagar, but God declared to him that the heir will come through Sarah. His faith was realized with the birth of Isaac at the ripe old age of 100 years old. For Abram it was a moment of belief in God’s promise to him as he looked at the stars that God made. It was a forward looking faith. It is prophetic as a promise to Abram.
Genesis 15 says this, “1 After this, the word of the LORD came to Abram in a vision: “Do not be afraid, Abram. I am your shield, your very great reward.” 2 But Abram said, “Sovereign LORD, what can you give me since I remain childless and the one who will inherit my estate is Eliezer of Damascus?” 3 And Abram said, “You have given me no children; so a servant in my household will be my heir.” 4 Then the word of the LORD came to him: “This man will not be your heir, but a son who is your own flesh and blood will be your heir.” 5 He took him outside and said, “Look up at the sky and count the stars—if indeed you can count them.” Then he said to him, “So shall your offspring be.” 6 Abram believed the LORD, and he credited it to him as righteousness.” (Read & Study The Bible – Daily Verse, Scripture by Topic, Stories). Citations are in the New International Versison.
James 2 cites Genesis 15 the writer in the present affirms the past faith of Abraham in believing God. The willingness of Abram to sacrifice his son, Isaac illustrates his faith in God even as he believed that God can raise his son from the dead. That is amazing faith. Abraham didn’t shirked or balked in his faith. He lived what he believed.
As one reads the broader context of faith in action, James says this, “You believe that there is one God. Good! Even the demons believe that—and shudder. 20 You foolish person, do you want evidence that faith without deeds is useless? 21 Was not our father Abraham considered righteous for what he did when he offered his son Isaac on the altar? 22 You see that his faith and his actions were working together, and his faith was made complete by what he did. 23 And the scripture was fulfilled that says, “Abraham believed God, and it was credited to him as righteousness,” and he was called God’s friend. 24 You see that a person is considered righteous by what they do and not by faith alone. 25 In the same way, was not even Rahab the prostitute considered righteous for what she did when she gave lodging to the spies and sent them off in a different direction? 26 As the body without the spirit is dead, so faith without deeds is dead” (NIV).
One can conclude that all promises of God are futuristic and thus they can be categorized as being prophetic. Prophesy can be defined as a declaration of God that may include aspects of futuristic events. It can be both forth telling and or foretelling. As one looks at the declaration of God to Abram, it is both forth telling and foretelling of his coming descendants.
SUMMARY: Genesis 15:6 is prophetic for it is both forth telling and foretelling to Abram of his coming descendants as the stars in the night sky.
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