A.T. Robertson Commentary Jude 1:20

"But ye, beloved, building up yourselves on your most holy faith, praying in the Holy Spirit," — Jude 1:20 (ASV)

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.