Building up (epoikodomountes). Present active participle of epoikodomeō, old compound with metaphor of a house (oikos), common in Paul (1Co 3:9-17; Col 2:7; Eph 2:20).
On your most holy faith (tēi hagiōtatēi humōn pistei). For the spiritual temple see also 1Pe 2:3-5. See pistis (faith) in this sense (cf. Heb 11:1) in 2Pe 1:5 with the list of graces added. A true superlative here hagiōtatēi, not elative.
Praying in the Holy Spirit (en pneumati hagiōi proseuchomenoi). This is the way to build themselves up on their faith.