As a bethel elder once said to me, "We put our faith and wish to be identified with, the God of the OT, with jehovah and not with anything from the NT".
Quite true. JWs are trying to be made righteous by following the law. They need to learn about God's grace and focus on some verses like these
Romans 3:20 (New International Version)
20 Therefore no one will be declared righteous in his sight by observing the law; rather, through the law we become conscious of sin.
Romans 9:30-33 (New International Version)
30 What then shall we say? That the Gentiles, who did not pursue righteousness, have obtained it, a righteousness that is by faith; 31 but Israel, who pursued a law of righteousness, has not attained it. 32 Why not? Because they pursued it not by faith but as if it were by works. They stumbled over the "stumbling stone." 33 As it is written:
"See, I lay in Zion a stone that causes men to stumble
and a rock that makes them fall,
and the one who trusts in him will never be put to shame." JWs are just like the Israel of old, they try to be justified by works rather than faith. They stumble over the One their faith should be in, the Rock that is Jesus. And this is made very clear by the name that Rutherford chose for them, along with the vainglorious ruse to put the name Jehovah all over the NT.
So true, they are living observing the law of the OT and have removed the name "New Testament" which points to the new covenant that is through Jesus. Hebrews 9:15 (New International Version)
15 For this reason Christ is the mediator of a new covenant, that those who are called may receive the promised eternal inheritance—now that he has died as a ransom to set them free from the sins committed under the first covenant.
The old covenant is obsolete and has been superseded
Hebrews 8:13 (New International Version)
13 By calling this covenant "new," he has made the first one obsolete; and what is obsolete and aging will soon disappear.
There is one covenant, one hope, one faith and one Mediator
Hebrews 12:24 (New International Version)
24 to Jesus the mediator of a new covenant, and to the sprinkled blood that speaks a better word than the blood of Abel.