The King James Bible uses “sanctify” to describe the process of something or someone being selected, or more accurate, of being set aside for holy or spiritual purposes, in short God’s purpose. Somebody or something that is sanctified, is set apart or even seperated from other people or things or concepts, because of being used or ordained for God’s own purpose. This state is called “being holy” or “holiness”.

The process to become “holy” is called “sanctification” or “being sanctified”.

Therefore the German translation of sactifiy should more refer to the process and be distinguishable from the state.

Because of this the German translation of the King James Bible is using “weihen” as a translation for “to sanctify”, instead of “heiligen”. In German “weihen” means to assign a religious purpose to something or somebody. The Catholic Church, not taking care of what the bible says, lets its officials “sanctify” people and things (places, buildings, names a.s.o.). But this idea is, of course, completely unbiblical.

Only God himself can sanctify somebody or something, making it holy in the process. In the King James Bible you will only see God or Jesus or the Holy Spirit sanctify anybody or anything. We humans can pray to God, pleasing him to sanctify somebody, admitted. But we can not in the slightest do anything about God’s decision who or what will be sanctified and therefore become holy.

Further: If something or somebody is “holy”, it has a special meaning to God. It has been assigned to serve for his purpose on his behalf.

So how do we translate John 17:19 then?

“And for their sakes I sanctifie my selfe, that they also might be || sanctified through the trueth.”

Luther and Schlachter translate this to

“[Und] ich heilige mich selbst für sie, damit auch sie geheiligt seien in Wahrheit.”

But this takes away the original meaning of this verse in German. Jesus is holy already. He doesn’t need to “make himself holy”; it just wouldn’t make any sense to make somebody holy who is already holy.

But this verse shows us, why the KJV translators used “to sanctify” here to translate ἁγιάζω (hagiazō): It’s kind of a little word play he is doing here. He “sanctifies himself” (from the world). So he separates himself from the world, meaning he leaves it, so that his disciples, the people that believe in him, are also separated from the world; just that for his disciples he doesn’t mean them to be taken out of the world (die), but being separated from the worldly people and things. In John 17:15 he makes it very clear, that he distinguishes here between him being separated from the world (dying) and his disciples being separated from the world (keeping them away from evil).

Verse 15 therefore is the key to how to translate “sanctify” with “weihen”. Jesus is dedicating himself to something, so that his disciples have advantage from that (being separated from evil, while still living in this world).