Does the New World Translation give accurate translations of Bible verses pertaining to the Christian's personal relationship with Jesus Christ?
1 Cor 1:9-God is faithful, by whom you were called into a sharing [Gr. fellowship] with his Son Jesus Christ our Lord.
2 Cor 11:3-But I am afraid that somehow, as the serpent seduced Eve by its cunning, your minds might be corrupted away from the sincerity and the chastity that are due the Christ [Gr. the simplicity and the purity in Christ].
2 Cor 13:5-Or do you not recognize that Jesus Christ is in union with you [Gr. Jesus Christ is in you]?
1 John 1:3-Furthermore, this sharing [Gr. fellowship] of ours is with the Father and with his Son Jesus Christ.
Reply: 1 Corinthians 1:9 the latest edition of the NWT does use the word "fellowship" with the word "sharing" in the footnote. A quick check at Vine's Expository Dictionary will let you know that the Greek word used here, KOINONIA can mean "sharing in common" as well as "fellowship". The Bible in Basic English, Revised English Bible, Simple English Bible, Moffatt's Bible, and the New English Bible read similarly to the older NWT.
As far as 2 Cor 11:3, few Bibles read exactly like you state, the American Standard Version being one I could find. The newer NWT uses the word "purity" in the footnotes. Many others use the word "sincerity" or "sincere" just like the NWT (see NIV, RSV, Amplified, NAB) and as for chastity/purity, see CHASTE in Vine's Dictionary and also Weymouth, Smith&Goodspeed (fidelity) and Rotherham (chasteness).
As far as using the words "in union with" see my post on this at: The New World Translation Bible and the Words "In Union With" and you will see that this has great support.
The Revised English has a nice comment on this: The phrase “in union with” is the translation of the Greek word en...We agree with Lenski (The Interpretation of St. Paul’s Epistles to the Galatians, Ephesians, and Philippians), that the en in this verse is used in its “static” sense, indicating a relationship: a union with, or a connection with, and that it is not an “instrumental use of en,” meaning “through” or “by way of” (see also W. Hendriksen, New Testament Commentary and Hawthorne and Martin, Word Biblical Commentary). We are “in Christ,” that is, “in union with Christ,” or “connected to” Christ, by virtue of being a member of his Body and identified with him. We feel that the verse is saying that we are strong by virtue of being in union with Christ rather than we are strong “through” Christ.
The static sense of en (“in”) is important in the New Testament but not well understood by most English readers. We understand the normal sense of “in,” and know what it is to be “in” a boat, “in” a house, or even “in the night” (Matt. 4:21; 5:15; John 11:10). But what does it mean to be “in Christ,” “in the Lord,” or “in him?” It means to be in connection with, in relationship with, or in union with. We feel “in union with” is the best English translation in this context because of its other uses in the New Testament.
https://www.revisedenglishversion.com/Philippians/chapter4/13
The PDF at https://www.wenstrom.org/downloads/written/word_studies/greek/en.pdf states that "in union with (association), (is) a marker of close personal association."
As far as 1 John 1:3, the latest edition of the NWT does use the word "fellowship" here.