There is no translation warrant for rendering Col 1:27-28 the way that the New World Translation does. That is because the Greek text does not have any words in those verses that could be translated as “union with”. The text simply says “in you, and “in Christ”. In order to produce the NWT rendition, they have had to add words in English that are just not in the Greek text.
Let me be clear: this is less a matter of translation, more a matter of addition. That additions have been inserted into the English rendition by those producing the NWT is clearly seen in their own Kingdom Interlinear Translation of the Greek Scriptures (KIT). There the Greek text (of Westcott & Hort) is reproduced, with a literal English translation under the Greek. In the adjoining column is the NWT English rendition. Go to pages 897 & 898 of that 1969 edition and it is obvious that words have been added in English that simply are not there in the Greek text.
Here is the literal English translation of the Greek text: 27 “to whom willed the God to make known what the riches of the glory of the mystery this in the nations, which is Christ in you, the hope of the glory;” 28 “whom we are announcing down putting mind into every man and teaching every man in all wisdom, in order that we might present every man perfect in Christ;”
Now here is the KIT rendition of those same verses: 27 “to whom God has been pleased to make known what are the glorious riches of this sacred secret among the nations. It is Christ in union with you, the hope of [his] glory.” 28 “He is the one we are publicizing, admonishing every man and teaching every man in all wisdom, that we may present every man complete in union with Christ.”
Note that the KIT puts ‘his’ in square brackets, to denote an added word, but does not do so for the two times ‘union with’ occurs.
The Theological Implications: The reasons why the NWT adds those words is not being asked for here. They have their reasons, of course. But because you asked what the theological implications are, then explanation needs to be made of how NWT readers who are Jehovah’s Witnesses could be impacted, theologically. Non-JW readers might not appreciate the subtle implications of those added words, unless they know that JWs believe only 144,000 people from the time of Christ are indwelt with the Holy Spirit, who is “in” the believer. Nearly all of the N.T. is addressed to Christians who are in the new covenant, and the evidence that they are, is stated in 2 Corinthians 13:5, where the test of a Christian is how they recognise that Jesus Christ is in them (and that is via the indwelling Holy Spirit). Ah, but the NWT adds those two little words again! The KIT reads, “Keep testing whether you are in the faith, keep proving what you yourselves are. Or do you not recognize that Jesus Christ is in union with you? Unless you are disapproved.”
It’s the same with Romans chapter 8 where the Holy Spirit indwelling the believer is every Christian’s guarantee that now they belong to Christ and will be with him in heavenly glory. Yet the KIT reads, “Therefore those in union with Christ Jesus have no condemnation. For the law of that spirit which gives life in union with Christ Jesus has set you free from the law of sin and of death… [vs 10] But if Christ is in union with you…”
But for verse 9 the KIT correctly translates exactly what the Greek text says, “However, you are in harmony, not with the flesh, but with the spirit, if God’s spirit truly dwells in you. But if anyone does not have Christ’s spirit, this one does not belong to him.” Then comes verse 10, as above.
Because 99% of all JWs are stated by their Governing Body not to be in the new covenant; not indwelt with the Holy Spirit, Romans 8 raises the question, then who do those ones belong to, if not Christ? Only those who have the Spirit of God (which is equally the Spirit of Christ) dwelling in them belong to Christ. Only they have no condemnation. But once the KIT has added those little words, the idea becomes that all who are in union WITH Christ belong to him and have no condemnation. And being in union with a person is different to being united with Christ by his indwelling Spirit! JWs suppose that they can still be in union with Christ without being indwelt with Christ (2 Cor 13:5 again.)
Theologically, 99% of the JW membership think those verses DO apply to them (though they happily admit they are not anointed with what they call ‘holy spirit’, and that only this 144,000 class are in the new covenant) so they read the NWT as saying they are “in union with Christ”. They take that to mean an agreement with Christ, united to him by faith in him, yet without the Holy Spirit having anointed and ‘sealed’ them. But they understand that to mean that they are equally in union with Christ as are the 144,000 because they have lost the impact of what it actually means to “be in Christ, the hope of glory” with Christ, in heaven as kings and priests alongside him. That 99% have no hope of heavenly glory being with Christ, ruling as kings and priests. However, if their NWT did not add that words that it does, they might see how it is only those who will be in glory with Christ in heaven who pass the test of 2 Corinthians 13:5. That is the theological danger of adding words to such texts – people supposing they can be “in union with Christ” without having Christ’s Holy Spirit anointing them.
Does any other Bible translation say “in union with Christ” instead of “in Christ”? As has already been stated, it appears that only the Good News Bible sometimes adds the words ‘union with’. Yet it does not make those additions as often does the NWT. In Col. 1:27 it simply says “Christ is in you” but in verse 28 it adds “union with”. In Romans chapter 8 the GNB says “union with” in verse 1 but not elsewhere in that chapter. In 2 Cor. 13:5 it sticks to the text and says, “Surely you know that Christ Jesus is in you?”