Recorded in Matthew 5:17, Jesus’ mission statement links the law, the prophets, and the fulfillment of both. In Jewish thinking, the term, law, brings to mind the first five books in the Old Testament—the Pentateuch: Genesis, Exodus, Leviticus, Deuteronomy, and Numbers. The term, prophets, loosely refers to the remainder of the Old Testament. In other words, Jesus takes as his task to fulfill all of the Old Testament scripture.
Law. The word, law, is often short for Law of Moses. Because “poor in spirit” can mean humble, Numbers 12:3 comes to mind. It reads: “Now the man Moses was very meek, more than all people who were on the face of the earth.” (Num 12:3 ESV). The verses that follow set Moses spiritually apart from both Aaron and Miriam, Moses’ brother and sister, because he had a unique relationship with God—one that exceeds the relationship of a normal prophet . The Hebrew word here (ana), translated as meek, can also be translated as poor, afflicted, humble, or meek .
Two important points follow from this word association. First, poor in spirit meaning humble draws us uniquely closer to God—Moses close. God spoke to Moses directly, face to face, not in riddles or dreams (Num 12:6-8). This is like a return to the Garden of Eden in terms of intimacy with God—God our father in heaven close. Second, in case you missed it in the first Beatitude, Jesus uses the word, meek, a second time in the third Beatitude. If he were speaking Hebrew, then he could have used the same word twice—an emphatic statement. The blessing of poor in spirit was: the kingdom of heaven. The blessing for meek was: inheriting the earth. What does the Bible start? “In the beginning, God created the heavens and the earth.” (Gen 1:1 ESV). In other words, being poor in spirit or meek in God’s eye gets you heaven and earth.
Of course, the opposite of humble is proud. While there are a lot of proud rulers in the Old Testament, Pharaoh is the archetype of a proud ruler, especially when you are thinking of Moses. What does God say through Moses to Pharaoh? Moses said: “Thus says the LORD, the God of the Hebrews, How long will you refuse to humble yourself before me? Let my people go, that they may serve me.” (Exod 10:3 ESV) Pharaoh refused and things ended badly for him .
Prophet. While Matthew 5:17 is a quite general statement of Jesus’ intent to fulfill all of scripture, Luke 4:18-19, which records Jesus’ call sermon, quotes almost verbatim from Isaiah 61:
“The Spirit of the Lord GOD is upon me, because the LORD has anointed me to bring good news to the poor; he has sent me to bind up the brokenhearted, to proclaim liberty to the captives, and the opening of the prison to those who are bound; to proclaim the year of the LORD’s favor, and the day of vengeance of our God; to comfort all who mourn; to grant to those who mourn in Zion…” (Isa 61:1-3 ESV)
When Isaiah writes about bringing “good news the poor”, he uses the same Hebrew word for poor (ana) as used in Numbers 12:3 . This passage is significant for at least two reasons. First, the use of the word, anointed, flags this passage as a messianic prophesy. Second, one might also ask whether the term, “broken-hearted”, is actually the better analogy to “poor in spirit” than “poor”. This suggests that a Isaiah 61 is indeed an important source not only for his call sermon but also for the Beatitudes.
Fulfillment. The word for fulfillment in the Greek text here (πληρόω) is generally translated as meaning: to bring to a designed end, fulfill a prophecy, an obligation, a promise, a law, a request, a purpose, a desire, a hope, a duty, a fate, a destiny (BDAG 5981, 4b). It was common for rabbis in Jesus’ day to preach from the law using the prophets to interpret what was meant. One might then perhaps say that the law had been “fulfilled” in following it correctly. However, the Gospel of Matthew uses fulfill more frequently than the other Gospels  and most often in reference to the fulfillment of prophecy (12/17), not mere compliance with law. In other words, for Matthew the focus in fulfillment is an action—to live out the prophecy in the sense of taking the next steps .
The law and the prophets are fulfilled in the faithful life, death, and resurrection of Jesus Christ. Jesus honors the poor in spirit who follow his lead in life, death, and eternal life.
 “And he said, Hear my words: If there is a prophet among you, I the LORD make myself known to him in a vision; I speak with him in a dream. Not so with my servant Moses. He is faithful in all my house. With him I speak mouth to mouth, clearly, and not in riddles, and he beholds the form of the LORD. Why then were you not afraid to speak against my servant Moses?” (Num 12:6-8 ESV)
 (עָנָי; BDB, 7237).
 “The waters returned and covered the chariots and the horsemen; of all the host of Pharaoh that had followed them into the sea, not one of them remained.” (Exod 14:28 ESV)
 The Greek Septuagint also uses the same word for poor as in Matthew 5:3 (πτωχοῖς (Isa 61:1 BGT)).
 Matthew [17 times] 1:22; 2:15, 17, 23; 3:15; 4:14; 5:17; 8:17; 9:16; 12:17; 13:35, 48; 21:4; 23:32; 26:54, 56; and 27:9. Mark [5 times]1:15; 2:21; 6:43; 8:20; and 14:49. Luke [7 times] 1:20; 3:5; 4:21; 7:1; 21:24; 22:16; and 24:44. John [15 times] 1:16; 3:29; 7:8; 12:3, 38; 13:18; 15:11, 25; 16:6, 24; 17:12-13; 18:9, 32; and 19:24, 36.
 Guelich (1982, 163) sees Jesus, for example, fulfilling Jeremiah 31:31-34 where God promises to write the law on our hearts.
BibleWorks. 2011. Norfolk, VA: BibleWorks, LLC. <BibleWorks v.9>.
Brown-Driver-Briggs-Gesenius (BDB). 1905. Hebrew-English Lexicon, unabridged.
Guelich, Robert. 1982. The Sermon on the Mount: A Foundation for Understanding. Dallas: Word Publishing.