That is why I told you is more literally “therefore (Greek oun) I told you.” Since this verse lays the basis for Jesus’ affirmation in verse 23, Good News Translation introduces it by the words That is why. New English Bible and New American Bible also handle the text in this way.

As indicated in the comments on verse 21, the Greek has the plural sins here, while there it has the singular “sin.” Good News Translation translates by sins in both places. Here the reference is not solely to the cardinal sin of rejecting Jesus, but to the totality of sins that these people have committed. In some languages you will die in your sins may be rendered “You will die with your sins” or “… with your sinning.”

On the meaning of the phrase I Am Who I am, see 4.26. In this passage, as in some others in John’s Gospel, the statement “I Am” is used absolutely, without a predicate. In such instances this expression identifies Jesus with God. The Greek order of this last sentence is the reverse of the order in Good News Translation (“for if you do not believe that ‘I Am,’ you will die in your sins”).

In some languages it is almost meaningless to say “I am who I am.” One may be obliged to say “I am really the person who I am” or “I am the one I claim to be.” Since such a translation obviously obscures the underlying allusion, it may be necessary to introduce a marginal note to explain the meaning which underlies the form of the Greek text.

Quoted with permission from Newman, Barclay M. and Nida, Eugene A. A Handbook on the Gospel of John. (UBS Handbook Series). New York: UBS, 1980. For this and other handbooks for translators see here .