Loose analogy of the OT/NT using math
For the purposes of this post OT means “Old Testament” and NT means “New Testament”.
Rejecting the OT is like saying there is no longer need to use addition or subtraction since we have multiplication, division, and algebra which are far superior for solving big problems. (And indeed they are superior for certain problems, but that doesn’t mean we don’t need the foundation too.)
Rejecting the NT is like rejecting multiplication and division because the same problem can (somewhat-but-not-really), supposedly be worked out on your own with enough effort using addition and subtraction.
Of course, this is just a loose analogy, but the point is: we need both.
The OT is the foundation, the way to walk and live our lives, which the NT expands on and explains and reveals in greater detail. The NT tells how we cannot solve our sin problem on our own. We need Yahushua who truly is superior to our filthy rags of righteousness.
If we throw out either, we are throwing out it all.
Yahushua said unto him, Thou shalt love Yahweh thy Elohim with all thy heart, and with all thy soul, and with all thy mind. This is the first and great commandment. And the second is like unto it, Thou shalt love thy neighbour as thyself. On these two commandments hang all the law and the prophets.
Matthew 22:37-40
So, I suggest that we choose to believe in the unified Word of truth which tells a consistent message from one end to the other. :)