In this verse God assures his people that he will be with them whenever they face danger, so that it does not overcome them.

When you pass through the waters I will be with you: Just as God was with Moses and the Israelites when they crossed the Reed Sea (or, Red Sea), so he will be with them when they face any other dangers, including any during their return from exile in Babylonia. The subordinate conjunction When introduces a possibility, so it may be rendered “Whenever” or “If.” Waters is a metaphor for any dangers the Israelites will face. I will be with you recalls the promise God made to Moses in Exo 3.12. An alternative rendering for this whole line is “Whenever you have to pass through deep water, I will be with you.”

And through the rivers, they shall not overwhelm you: This line is parallel to the previous one. Here God promises his people that their troubles will not defeat them. The phrase When you pass is implied here. Rivers is another metaphor for dangers, so it may be rendered “wide/deep rivers” to make this clear. Overwhelm you means “to flow over you,” implying drowning. The Hebrew verb rendered overwhelm is the same one translated “overflow” in 8.8, where it refers to destruction.

Translators may change the order of waters and rivers in the first two lines for naturalness. Good News Translation combines them by saying “deep waters” in its first line. Then its next line makes it explicit that these are metaphors for “your troubles.” This model may be helpful for other languages.

When you walk through fire you shall not be burned, and the flame shall not consume you: These two synonymous parallel lines use the metaphors of fire and flame to represent dangers (compare 41.25). Just as Daniel’s friends were not burned in the furnace (Dan 3.27), so God will protect his people from other dangers. As in the first line, the subordinate conjunction when introduces a possibility. The Hebrew verb rendered consume means “to burn up completely.” Revised Standard Version uses the imagery of eating, which fits this context well. In Hebrew there is a sound play between this verb, which is thivʿar, and the verb for pass through, which is thaʿavor. Good News Translation combines fire and flame into “fire,” and it also makes it explicit that these are metaphors for “hard trials that come.”

For the translation of this verse consider the following examples:

• When you cross the waters, I will be with you;
and when you cross the rivers, you will not drown.
When you pass through fire, you will not be burned,
and the flames will not devour you.

• Whenever you pass through deep water, I will be there with you;
and whenever you cross deep rivers, they will not sweep you away.
Whenever you go through fire, it will not burn you,
and the flames will not consume you.

Quoted with permission from Ogden, Graham S. and Sterk, Jan. A Handbook on Isaiah. (UBS Helps for Translators). New York: UBS, 2011. For this and other handbooks for translators see here .