The fact of the matter is, there are conflicts, inconsistencies, and incompatibilities between the Old Testament and the New Testament (if there wasn’t we’d have never heard of Gnosticism), most especially when Christians narrowly focus only on the bible’s words as ultimate and absolute authority.*
Admittedly there are conflicts and incompatibilities between the Bible and the Qur’an as well*, but, once again, only when viewed (by both sides) through the paradigm of a book as being the source of authority and not God.
The real issue isn’t whether the Torah of the Old Testament and the New Testament are not compatible, nor whether the bible and the Qur’an are in conflict and suffer incompatibility. We are misled and misdirected when we focus on and come into conflict over these issues. We fall victim to pride and seek to answer the question of whose right rather than listening to the voice of the Spirit.
When we decide to follow the Way of Yeshua (or the Holy Spirit for that matter) we must decide whether
A) we will narrowly focus only on a book’s words as our pinnacle of authority, or
B) allow the principles of the Spirit beneath the words to be our guiding force.
It is these two possibilities that are in conflict. It is these two options that are totally incompatible. There is no happy medium between these two points.
You deliberately follow the letter of the law, or you deliberately follow the spirit of the law. You may only do both incidentally. One will shackle and bind you while the other will set you free – whether Jew, Christian, Muslim, freeman, master, or slave.
And it is important to make note that Yeshua was very specific about disregarding the will and direction of the Holy Spirit.
* I suspect these incompatibilities stem from different and varying authors, historic contexts, and cultures btw.