John 17:17

[Van]

EHV
“Sanctify them by the truth. Your word is truth.
ESV
Sanctify them in the truth; your word is truth.
NET
Set them apart in the truth; your word is truth.
NLV
Make them holy for Yourself by the truth. Your Word is truth.

Here we have four English translations of John 17:17.

Sanctify either means to consecrate/set apart on one hand, and make them holy on the other.
Looking at John 17:19 I think the idea is to separate or protect His disciples from evil.

Next we have God's word having the instrumentality to set them apart, or the disciples are to be set apart in God's word.
However, they have already been given God's word (John 17:14) so the idea seems to ask for God's power to keep them "in God's word"
such that they replicate Christ's message in a clear and believable fashion.

 

[Van]

Set my disciples apart in the truth of your word, such that they spread the gospel in a clear and believable fashion.

Not what the verses literally says, but perhaps God's intended message. Or so I believe.

 

[Van]

As we know, scripture tells us to live by God's word, to meditate upon it and study it and be diligent to rightly understand it.

Thus the application of John 17:17 is to do our best to stay within the actual message of scripture and not to depart due to a lack of study, or due to pride, confirmation bias, or the traditions of men.