In recent days a number of strange claims have been made about slavery and shellfish in the Bible.

The line normally goes something like this: although the Bible prohibits God's people from eating shellfish and also endorses slavery, we can disregard these ethical instructions because we have come of age and can see things differently - indeed, more clearly - with our advanced knowledge and superior wisdom concerning what is right and wrong. Therefore, when it comes to novel concepts such as redefining marriage to include two persons of the same sex, we can simply abandon the teaching of the Bible, and in particular, even the teaching of Jesus, on the grounds that the Bible has been superseded by the moral insights of the twenty-first century.

This confused way of handling the Bible springs from an ignorance of the Bible's own narrative. The Bible's story is a progressive one, unfolding through the lives of Noah, Abraham and Moses (and the nation of Israel) and culminating in the arrival of Jesus, the long awaited Messiah, not only of the Jewish people, but of all people - from every tribe and nation.

In preparation for the coming of Jesus, God provided specific cultic commands for the nation of Israel as a visual teaching aid for understanding holiness of life through ceremonies of ritual cleanness, which specifically distinguished Israel from other nations. An obvious example is the system of sacrifices instituted under Mosaic law, and the corresponding distinctions between clean and unclean food - hence the prohibition of shellfish. Yet, these only applied when God's people were co-extensive with the nation of Israel (while also including any non-Israelite who wanted to follow the God of Israel), which identified them as being both morally and ceremonially distinct from all other nations.

However, when Jesus arrives, he comes to fulfil the law of Moses (Matthew 5:17). A significant consequence of his coming is the fulfilment of God's promise to Abraham that all nations would be blessed, without needing to attach themselves to the Jewish nation. Consequently, the need for national identity markers, such as food laws and circumcision, are no longer valid under the new covenant, which is established by Jesus. This is foretold by Jesus's own teaching in Mark 7:19 and expounded by the apostle Paul in 1 Corinthians 7:19. The teaching methodology of ritual cleanness is thereby abolished, along with animal sacrifices and food laws, because these symbolic markers have found their fulfilment in the life and death of Jesus, who is the way, the truth and the life.

That the Bible commands a diet of only ceremonially clean food at one stage of redemptive history and then abandons this requirement when Jesus comes to fulfil God's purposes for humankind is not some form of contrariness, or worse, an inherent contradiction in the Bible's teaching. Rather, it is part of God's intended plan in preparing his people for the coming of the Messiah Jesus. The apostle Paul likens this transition to that of a minor coming of age (Galatians 4:1-7). It reflects the unfolding purposes of God's plan through the distinctive ages of human existence.

Therefore, it is a shallow approach to the Bible to mock the prohibition concerning the eating of shellfish (Leviticus 11:9-12) as if it still applied today, without understanding this temporary command within the sweep of redemptive history and the explicit teaching of Jesus who has come to liberate us from such ceremonial and cultic behaviour which distinguishes between clean and unclean foods.

Moreover, it is also a misguided approach to the Bible's teaching to infer that because the form of ceremonial activity has changed, that the ethical imperative undergirding the ceremony has also changed. Not so! Jesus's words in Mark 7:18-23 are as instructive to us today as they were to his first century hearers:

"Do you fail to understand?" Jesus asked. "Don't you see that nothing that enters a person from the outside can defile them? For it doesn't go into their heart but into their stomach, and then out of their body." (In saying this, Jesus declared all foods clean.) He went on: "What comes out of a person is what defiles them. For it is from within, out of a person's heart, that evil thoughts come - sexual immorality, theft, murder, adultery, greed, malice, deceit, lewdness, envy, slander, arrogance and folly. All these evils come from inside and defile a person."

Jesus affirms the moral integrity of God's laws expressed in the Old Testament, and their abiding character and application for us today, but re-establishes them for his disciples in a non-ceremonial and non-cultic manner, as befits the age of fulfilment that Jesus came to bring.

When Jesus taught his disciples about the sanctity of marriage, he reminded them that marriage was not a human invention but God's idea: an exclusive relationship between a man and a woman for life. Yet he also recognised that in a fallen and broken world, some marriages may end in divorce, due to the unfaithfulness of one or both parties. While this was not the original intention, Moses's law provided for divorce in certain circumstances, and so did Jesus.

In similar manner, the Old Testament provided for the equitable treatment of slaves, but this was not part of God's original design, where all men and women were created equal. That Israelites could not be kept in slavery for more than six years (Exodus 21:2) demonstrates that even in a broken world, God saw slavery as temporary, and the redemption of the Israelites from their slavery in Egypt bears ample testimony to God's purposes for bringing freedom from bondage for all humankind and his condemnation of the slave trade (1 Timothy 1:10).

While Australians wrestle with the implications of redefining marriage to include a union of two persons of the same sex, it would be a much more enlightened debate if proponents of this novel redefinition did not misuse the Bible in mounting their arguments. It would be more honest to declare their disagreement with biblical teaching, rather than pretend by shallow, ill-informed exegesis that they are following the Bible's primary theme of love. Here again, Jesus's words are instructive: "If you love me, keep my commandments" (John 14:15).

Glenn Davies is the Archbishop of Sydney and Metropolitan of New South Wales in the Anglican Church of Australia.