John 17:17

The next thing he prayed for for them was that they might be sanctified; not only kept from evil, but made good. I. Here is the petition (Joh 17:17): Sanctify them through thy truth, through thy word, for thy word is truth; it is true - it is truth itself. He desires they may be sanctified, 1. As Christians. Father, make them holy, and this will be their preservation, Th1 5:23. Observe here, (1.) The grace desired - sanctification. The disciples were sanctified, for they were not of the world; yet he prays, Father sanctify them, that is, [1.] "Confirm the work of sanctification in them, strengthen their faith, inflame their good affections, rivet their good resolutions." [2.] "Carry on that good work in them, and continue it; let the light shine more and more." [3.] "Complete it, crown it with the perfection of holiness; sanctify them throughout and to the end." Note, First, It is the prayer of Christ for all that are his that they may be sanctified; because he cannot for shame own them as his, either here or hereafter, either employ them in his work or present them to his Father, if they be not sanctified. Secondly, Those that through grace are sanctified have need to be sanctified more and more. Even disciples must pray for sanctifying grace; for, if he that was the author of the good work be not the finisher of it, we are undone.

— Matthew Henry's Concise Commentary (public domain)

Sanctify them--As the former prayer, "Keep them," was "negative," asking protection for them from the poisonous element which surrounded and pressed upon their renewed nature, so this prayer, "Sanctify them," is positive, asking the advancement and completion of their begun sanctification. through--in. thy truth--God's revealed truth, as the medium or element of sanctification; a statement this of immense importance. thy word is truth--(Compare Joh 15:3; Col 1:5; Eph 1:13).

— Jamieson-Fausset-Brown Commentary (public domain)