The insertion of Jehovah into the NT is only understandable to me if there was an OT quote. However by far the vast majority of the time this is not the case. Instead we see clear bias in the NWT. Here is just one example:
First a simple question- Did Jesus know their hearts?
Here are a few verses that tell us the answer.
Mark 2:8 Immediately Jesus, aware in His spirit that they were reasoning that way within themselves, *said to them, “Why are you reasoning about these things in your hearts?
Luke 6:8 But He knew what they were thinking, and He said to the man with the withered hand, “Get up and come forward!” And he got up and came forward.
John 2:24-25 But Jesus did not commit Himself to them, because He knew all men, and had no need that anyone should testify of man, for He knew what was in man.
There are others, but I think that's enough. Hopefully, we agree that's a yes, Jesus knows man's heart.
Next question. Who chose the apostles? It's not a hard one so Hopefully we agree it was the Lord Jesus.
That takes me to Acts 1:21-26
Therefore it is necessary that of the men who have accompanied us all the time that the Lord Jesus went in and out among us— 22 beginning with the baptism of John until the day that He was taken up from us—one of these must become a witness with us of His resurrection.” 23 So they put forward two men, Joseph called Barsabbas (who was also called Justus), and Matthias. 24 And they prayed and said, “You, Lord, who know the hearts of all men, show which one of these two You have chosen 25 to occupy this ministry and apostleship from which Judas turned aside to go to his own place.” 26 And they drew lots for them, and the lot fell to Matthias; and he was added to the eleven apostles.
Who did the apostles pray to there? The obvious answer is the Lord (Jesus.)
So, why does the NWT insert Jehovah there? I will let you answer that in your heart.
Peace be with you all.