But ye, beloved, building up yourselves on your most holy faith, praying in the Holy Ghost, Building up yourselves - Having the most holy faith - the Gospel of our Lord Jesus, and the writings of his apostles, for your foundation; founding all your expectations on these, and seeking from the Christ who is their sum and substance; all the grace and glory ye need.
Praying in the Holy Ghost - Holding fast the Divine influence which ye have received, and under that influence making prayer and supplication to God. The prayer that is not sent up through the influence of the Holy Ghost is never likely to reach heaven.