Building up (εποικοδομουντες). Present active participle of εποικοδομεω, old compound with metaphor of a house (οικος), common in Paul (1. Corinthians 3:9-17; Colossians 2:7; Ephesians 2:20).

On your most holy faith (τη αγιωτατη υμων πιστε). For the spiritual temple see also 1. Peter 2:3-5. See πιστις (faith) in this sense (cf. Hebrews 11:1) in 2. Peter 1:5 with the list of graces added. A true superlative here αγιωτατη, not elative.

Praying in the Holy Spirit (εν πνευματ αγιω προσευχομενο). This is the way to build themselves up on their faith.