Why dopastors still preach teach and insist on Old Testament laws when the
same stand cancelled in the New Testament?

Preachers teach the need for Christians to follow the Old Testament law
because of bad theology. This is one of the most common false doctrines
in the “Christian” world. Different religious groups teach the need to
obey Old Testament commands to varying degrees. Perhaps the most blatant
example is the Seventh Day Adventist group which openly teaches that we
are still subject to the Mosaic covenant. They are very inconsistent in
this–requiring obedience to the Sabbath, certain food requirements and so
forth, but not adherance to skin disease rules and animal sacrifice and so
forth. Other religious groups command some sort of “Christian” Sabbath or
teach that the eating of blood is forbidden and so forth. The most common
version of incorporation Old Testament law into Christianity is those who
teach that the Ten Commandments are Christian law. This is perhaps a
relatively harmless (not completely harmless) teaching, as at least eight
or perhaps nine of the ten are implied or explicitly stated in the New
Testament as commandments of Jesus to his followers. It is ironic that
certain conservative Christian groups fight so hard to make public display
of the Ten Commandments an issue when they are not necessarily bound on
Christians.

All this is not much use, of course, unless one can back up the claim that
disciples of Jesus are not subject to the law of Moses. A little Bible
study is called for. A couple of relevant passages are Matthew 5:17, “Do
not think that I have come to abolish the Law or the Prophets; I have not
come to abolish them, but to fulfill them. Those who seek to incorporate
Old Covenant laws into Christianity often use this as a proof text that
the Old Testament Law is still in force, but that is exactly the opposite
of what Jesus is teaching here. He is saying that through his life, he
completely met the requirements of the Law; thus fulfilling it.
Ultimately even replaced replacing it with the Law of Christ. Other
passages are required to pull out the full meaning of Matthew 5:17. For
example, one can turn to Mark 7:17-19, in which Jesus officially declared
the dietary laws of Judaism to not apply to his followers. Perhaps more
significantly, one can find a very strong statement about the Law of Moses
in Colossians 2:13f (in fact, this is the main subject of the whole
book). “When you were dead in your sins and in the uncircumcision of your
sinful nature, God made you alive with Christ. He forgave us all our
sins, having canceled the written code, with its regulations, that was
against us and that stood opposed to us; he took it away, nailing it to
the cross. After this passage, Paul goes on to specifically mention the
Sabbath and other laws which are not in force for Christians. Many other
passages could be called on to support the thesis that we are under the
Law of Christ, not the Law of Moses. Romans and Hebrews are both full of
relevant passages. Given the nature of your question, I am guessing that
this is not entirely new to you.

That gets us back to why people still teach these things. Paul also
addresses this question in Colossians 2:16-23. It is human nature to
impose needless rules to help define what it means to be “good” or
spiritual. Paul roundly condemns the spirit of requiring people to follow
man-made rules or even laws of Moses which have passed away as useless to
create true spirituality. Many teach these things because it is all they
know and because they do not do the careful Bible study required for them
to leave their false teachings. I am not in a place to explain or to
judge the motives of all these teachers, and perhaps it is best not to do
so, but rest assured that disciples of Jesus do not need to follow the
rules and regulations of the Old Covenant unless they are specifically
repeated in the New Testament.

John Oakes