Building up yourselves; he compares them to a house, which is to be built up, whereof faith is the foundation: the same metaphor is used, 1 Corinthians 4:9 Ephesians 2:20-22 1 Peter 2:5. Most holy; so he calls faith, as being the means of purifying their hearts, and working holiness in them; and in opposition to the false faith of the heretics he warns them against, which did consist with so much impurity. Faith; this may be understood either:
1. Of the grace of faith; and then that is compared to the foundation, as being the first and principal grace in a Christian, and of greatest necessity and use; and then they are here bid to build themselves up in other graces which follow upon faith, as 2 Peter 1:5. Or:
2. Of the doctrine of faith, that on which their faith itself is founded; and then the meaning is, that they should not rest satisfied in what measure of faith they had already attained, but still be improving it, and making further progress in it, not only hold fast the truth of the gospel, the right foundation on which they had begun to be built, but get themselves, by the due study and meditation of the word, more and more confirmed in the belief of it. Praying in the Holy Ghost; i.e. by the assistance of the Spirit, who teacheth what to pray for, and how; from whom faith, fervency, and all praying graces do proceed. Romans 8:26,27: The Spirit maketh intercession (prays) in us, to note the excitations of his grace; here we are said to pray in the Holy Ghost, to note the concurrence of our faculties.