As a non Christian, it's very confusing how Christians see the Old Testament. Some say ignore it, there is a new Covenant. Some Christians choose certain parts of it, and some say the Laws still stand. So my question is, why is the Old testament still in the Bible if it's not relevant still? And, if it is still relevant, how do you choose what's relevant and what's not (mixed fabrics, eating seafood, gays)
The complicated relationship(s) which Christians have with the Old Testament largely comes down to significant differences between different traditions within modern Christianity.
So as a disclaimer: My views are going to represent a Lutheran, and more broadly, a traditional Christian perspective--something that I think would be agreed upon rather generally by the more historic, and traditional Christian churches, Catholic, Orthodox, Anglican, et al. I simply want to provide where I am coming from here; though I regard my position as correct, it's still important to confess my bias.
In the beginning of Christianity the only scriptures available are those which would eventually become known as the Old Testament. While there was no "Bible" to speak of, there was a loose acknowledgment of certain writings as holy within Judaism, specifically Pharisaic Judaism (this was the form of Judaism which met in the synagogues, had rabbis, and was more-or-less the mainstream Judaism of the regular Jew both in Judea as well as elsewhere in the Roman Empire). The Pharisees, in contrast with the other major Jewish sect the Sadducees, embraced not only the five books of the Torah, but also the group of writings known as "the Prophets", as well as the Psalms (and other books classified broadly as "the Writings"). The Sadducees, in contrast, only accepted the five books of the Torah, rejecting the works of the Prophets and Writings, and as such had quite different views than the Pharisees on things. The Pharisees believed in angels, the resurrection of the dead, etc; the Sadducees, on the other hand, did not. The Sadducees, however did not have much traction among the regular Jewish people, but was basically restricted to the priestly, aristocratic families in Jerusalem.
The closest thing Christians had to a "Bible" at the time was the Septuagint, a translation of the Jewish Scriptures into Greek made several centuries before Christ. The Septuagint was hardly a defining "Bible" in Judaism, but did provide a readily available set of Scriptures for early Christians who largely were communicating their faith to Greek-speaking Jews and Gentiles (Romans, Greeks, etc)
The Christian religion is centered on a very specific confession of faith: Jesus of Nazareth is the Christ, that is, the Messiah. When we say "Jesus Christ" we are saying "Jesus the Messiah". This messianic idea was borne out of Jewish hope for a deliverer, a king who would rise up from the line of David. Messianic ideas were often seen mentioned in the books of the Prophets, but there wasn't a universal messianic position.
However, in calling Jesus the Messiah the early Christians were arguing that Jesus was the promised figure mentioned by the Prophets, and indeed, came to read many of the Scriptures as preaching the Messiah. So in the Gospels we have Jesus saying, "You search the Scriptures because in them you believe you have eternal life, it is these which bear witness to Me" forms, in essence, the fundamental Christian exegesis of the writings which make up the Old Testament: that these writings, if read through the revelation of Jesus properly communicates Jesus to us. The writings which would eventually make up the New Testament, largely, are based upon this premise; they exist--in a sense--as early Christian commentary on the relationship between Jesus and the long history of God's work, in creation, the story of Adam, the call of Abraham, the giving of the Torah through Moses, the work of God through the Prophets and the long story of Israel ultimately building up, culminating, and having fulfillment in the coming of the Messiah.
Thus the significance of the Old Testament is this: It is the long story of God's redemption in history and for the world which finds its fulfillment in Jesus.
In other words, "You search the Scriptures because in them you believe you have eternal life, it is these which bears witness to Me." The Old Testament speaks of Christ, we read the Old Testament Christocentrically, Christ is the word and substance behind the letter of the Old Testament text. St. Augustine once said, "There is one Utterance in all of Scripture", by "Utterance" he means Jesus. Jesus is, as the prologue of John's Gospel says, "The Word", Jesus is God's Word made flesh, God's Revelation of Himself. Jesus is the Word in Scripture, the Scriptures contain the Word of God, Jesus, and that is what makes them
Holy Scripture; they are divinely inspired for this reason and purpose.
The covenants we read about in the Old Testament, such as those made to Abraham, Moses, and David, have their fulfillment in Jesus. So the giving of the Torah through Moses to Israel and the establishment of Israel as God's covenant people to observe and keep the Torah as a holy nation established by God is seen, in historic, traditional Christian teaching as serving the purpose of ultimately bringing God's Messiah, Jesus. The reason why Christians don't observe the commandments of the Torah is twofold:
1) The Torah was given exclusively to the Jewish people as part of the covenant established with them at Mt. Horeb at Sinai. It isn't for anyone else, it's not a universal set of instructions, but a particular set of instructions for a particular people with a particular covenant.
2) With the coming of the Messiah we believe that the former things have found their fullness and purpose in Him, and that God has established a universal covenant with the world through Jesus, that is what we mean by "new covenant"; this covenant is a covenant not based upon a particular law or nation, but on the promise of God's gift of freedom, redemption, resurrection, eternal life, and the renewal of all things through Jesus, both through His death and resurrection from the dead, the forgiveness of our sins, as well as the future redemption of all creation when Christ returns, the dead are raised, and God makes all things new in the Age to Come.
The Torah is, therefore, not a set of universal instruction, but instruction for a particular people; and which served the purpose of ultimately pointing to Jesus; chiefly because God called a covenant people through which would come the Messiah. That is the long story of redemption in the Old Testament: the creation, fall of man, the calling of Abraham, the making of the covenant nation, and ultimately Messiah Jesus through whom God's kingdom and purposes are manifest, displayed, and given for the whole world, now and also in the end at the consummation of all things.
That is how we read and use the Old Testament; as being Christ-bearing word which communicates hope and faith in Christ; directing us to our present hope and faith in Him, and of the promises which we have in Him both now as God's people in the world, and concerning our hope and the promises of future redemption for the whole of creation.
-CryptoLutheran