He is a faithful as well as a merciful high priest in God’s service. It is possible, of course, to suggest here that in His high priestly service Christ is merciful towards man and faithful to God, the twin title reflecting the manward and Godward aspects of His ministry.
But it is equally true that He is faithful to us. The trustworthy and reliable Christ is the One who comes to our help when we are tempted, not a vacillating, capricious, occasionally unavailable Helper, but One who has proved Himself fully dependable and completely adequate in every experience of life.
Raymond Brown, The Message of Hebrews, Bible Speaks Today, IVP, 1982 pg. 72