Jamieson, Fausset and Brown on Jude 1:20

20. Resuming Jude 17.

building up yourselves--the opposite to the "separate themselves" (Jude 19): as "in the Holy Ghost" is opposed to "having not the Spirit."

on--as on a foundation. Building on THE FAITH is equivalent to building on Christ, the object of faith.

praying in the Holy Ghost--(Ro 8:26; Ep 6:18). The Holy Spirit teaches what we are to pray for, and how. None can pray aright save by being in the Spirit, that is, in the element of His influence. Chrysostom states that, among the charisms bestowed at the beginning of the New Testament dispensation, was the gift of prayer, bestowed on someone who prayed in the name of the rest, and taught others to pray. Moreover, their prayers so conceived and often used, were received and preserved among Christians, and out of them forms of prayer were framed. Such is the origin of liturgies [Hammond].