ISB ~ Hebrews 3 v1

holy brethren:Believers are “holy” because they are sanctified (2:11) and “brethren” because they share in the Sonship of Christ, who humbled himself to share in our humanity (2:14, 17).

a heavenly call:A summons that comes from heaven and leads us to heaven (Phil 3:14).

the apostle:i.e., the One sent by God. This is the only time the NT gives this title to Christ, though the notion that Jesus is the One sent into the world by the Father is expressed in other terms (Mt 10:40; Lk 10:16; Jn 3:17, etc.).