Nevertheless, Christ himself explained the key to understanding the Old Testament in response to a question by one of the scribes/lawyers of the Pharisees. This account is found in Matthew and Mark. Since the Matthew account is more familiar to most people, I'll put it first, but note that the Mark account provides some additional detail that is not included in the Matthew account.
But when the Pharisees had heard that he had put the Sadducees to silence, they were gathered together. Then one of them, which was a lawyer, asked him a question, tempting him, and saying, "Master, which is the great commandment in the law?"Mark 12:28-33
Jesus said unto him, "'Thou shalt love the Lord thy God with all thy heart, and with all thy soul, and with all thy mind' (Deuteronomy 6:5; cf. Deuteronomy 7:9, 10:12, 11:1, 19:9; 30:6, 30:16, and 30:20). This is the first and great commandment. And the second is like unto it, 'Thou shalt love thy neighbour as thyself' (Leviticus 19:18; cf. Leviticus 19:34). On these two commandments hang all the law and the prophets."
And one of the scribes came, and having heard them reasoning together, and perceiving that he had answered them well, asked him, "Which is the first commandment of all?"Notice that the key to understanding the Old Testament, "all the law and the prophets," so that there is "none other" commandment, is love of God and love of one's neighbor. Thus, it is written: "love is the fulfilling of the law" (Romans 13:10).
And Jesus answered him, "The first of all the commandments is, 'Hear, O Israel; The Lord our God is one Lord: and thou shalt love the Lord thy God with all thy heart, and with all thy soul, and with all thy mind, and with all thy strength:' (Deuteronomy 6:4-5; cf. Deuteronomy 7:9, 10:12, 11:1, 19:9; 30:6, 30:16, and 30:20) this is the first commandment. And the second is like, namely this, 'Thou shalt love thy neighbour as thyself.' (Leviticus 19:18; cf. Leviticus 19:34). There is none other commandment greater than these."
And the scribe said unto him, "Well, Master, thou hast said the truth: for there is one God; and there is none other but he (cf. Deuteronomy 4:35, 1 Samuel 2:2, 2 Samuel 7:22, 1 Chronicles 17:20, Isaiah 45:5-6, and Isaiah 45:21): and to love him with all the heart, and with all the understanding, and with all the soul, and with all the strength, and to love his neighbour as himself, is more than all whole burnt offerings and sacrifices" (cf. Hosea 6:6).
If you want to properly understand the Old Testament, you need to keep this hermeneutical grid in mind as one of the primary hermeneutical grids. It is true that there is much about the person and work of Christ in the law and the prophets (Luke 24:44, John 1:45, Acts 13:15, 24:14, and 28:23), but when it comes to saying what "all the law and prophets" hang on, Christ taught that it was "Love God" and "Love thy Neighbor."
Scripture texts referenced above:
Hear, O Israel: The LORD our God is one LORD: and thou shalt love the LORD thy God with all thine heart, and with all thy soul, and with all thy might.
Know therefore that the LORD thy God, he is God, the faithful God, which keepeth covenant and mercy with them that love him and keep his commandments to a thousand generations;
And now, Israel, what doth the LORD thy God require of thee, but to fear the LORD thy God, to walk in all his ways, and to love him, and to serve the LORD thy God with all thy heart and with all thy soul,
Therefore thou shalt love the LORD thy God, and keep his charge, and his statutes, and his judgments, and his commandments, alway.
If thou shalt keep all these commandments to do them, which I command thee this day, to love the LORD thy God, and to walk ever in his ways; then shalt thou add three cities more for thee, beside these three:
And the LORD thy God will circumcise thine heart, and the heart of thy seed, to love the LORD thy God with all thine heart, and with all thy soul, that thou mayest live.
In that I command thee this day to love the LORD thy God, to walk in his ways, and to keep his commandments and his statutes and his judgments, that thou mayest live and multiply: and the LORD thy God shall bless thee in the land whither thou goest to possess it.
That thou mayest love the LORD thy God, and that thou mayest obey his voice, and that thou mayest cleave unto him: for he is thy life, and the length of thy days: that thou mayest dwell in the land which the LORD sware unto thy fathers, to Abraham, to Isaac, and to Jacob, to give them.
Thou shalt not avenge, nor bear any grudge against the children of thy people, but thou shalt love thy neighbour as thyself: I am the LORD.
But the stranger that dwelleth with you shall be unto you as one born among you, and thou shalt love him as thyself; for ye were strangers in the land of Egypt: I am the LORD your God.
Unto thee it was shewed, that thou mightest know that the LORD he is God; there is none else beside him.
1 Samuel 2:2
There is none holy as the LORD: for there is none beside thee: neither is there any rock like our God.
2 Samuel 7:22
Wherefore thou art great, O LORD God: for there is none like thee, neither is there any God beside thee, according to all that we have heard with our ears.
1 Chronicles 17:20
O LORD, there is none like thee, neither is there any God beside thee, according to all that we have heard with our ears.
I am the LORD, and there is none else, there is no God beside me: I girded thee, though thou hast not known me: that they may know from the rising of the sun, and from the west, that there is none beside me. I am the LORD, and there is none else.
Tell ye, and bring them near; yea, let them take counsel together: who hath declared this from ancient time? who hath told it from that time? have not I the LORD? and there is no God else beside me; a just God and a Saviour; there is none beside me.
For I desired mercy, and not sacrifice; and the knowledge of God more than burnt offerings.
And he said unto them, These are the words which I spake unto you, while I was yet with you, that all things must be fulfilled, which were written in the law of Moses, and in the prophets, and in the psalms, concerning me.
Philip findeth Nathanael, and saith unto him, We have found him, of whom Moses in the law, and the prophets, did write, Jesus of Nazareth, the son of Joseph.
And after the reading of the law and the prophets the rulers of the synagogue sent unto them, saying, Ye men and brethren, if ye have any word of exhortation for the people, say on.
But this I confess unto thee, that after the way which they call heresy, so worship I the God of my fathers, believing all things which are written in the law and in the prophets:
And when they had appointed him a day, there came many to him into his lodging; to whom he expounded and testified the kingdom of God, persuading them concerning Jesus, both out of the law of Moses, and out of the prophets, from morning till evening.
Love worketh no ill to his neighbour: therefore love is the fulfilling of the law.