Just some ‘thinking out loud.’
I note that in Hebrews, the adjectives merciful, faithful and sympathetic are used to describe Jesus Christ in His high-priestly vocation. The ‘faithfulness’ sources in a comparison between Jesus and Moses in Chapter 3. He is likened to the HighPriests as sympathetic in Chapter 5. In each of these He is shown to exceed His counterpart to the point of ultimacy. Jesus shows the ultimate in Faithfulness, the ultimate in Sympathetic.
There does not appear to me to be a point of comparison for “Merciful.” Moses is not described with that term, nor are the Aaronic priests.
I wonder, out loud, whether that is because the mercy evident as we look at the vocational activities of Moses and the Aaronic priests is God’s mercy. The redemption of Israel out of Egypt…the release of the Day of Atonement…there is mercy, but it is God’s mercy mediated through Moses and the Aaronic priests. It is not the mercy of the non-divine human mediators.
The mercy finds its source in Jesus. As our Author says, He is the source of salvation. So Jesus alone, in the context of Hebrews, can be described as merciful.
Again, just thinking out loud.