Judah was his sanctuary, and Israel his dominion.
Judah was his sanctuary. "Was" in Hebrew is feminine, whereas Judah is masculine. Judah is therefore here personified as a virgin (Isaiah 22:4), "the daughter of my people" (Psalms 45:12). As the description of God as "the holy One" denotes His separation far above every created being (Psalms 22:3), so the choice of Judah as "His sanctuary" or holy dwelling denotes the elect nation's separation from the world and consecration as "holy unto the Lord her God" (Deuteronomy 7:6; Exodus 19:5-6). God by acts of deliverance manifested His choice of the people before He in words declared it at Sinai. Judah here is made to represent the nation, as being, from the time of the carrying away of the Ten tribes, the surviving heir of the ancient promises belonging to it (Psalms 76:1).
Moreover, Judah from David's time was the seat of the royal family and of the national worship (Psalms 78:68-71). It the more strikingly, because undesignedly, shows the sense of God's continual presence which was realized by the Psalmist, that the "HIS" is introduced, though God had not been named before; as if every believer would instinctively know who was meant, and needed not to hear the name of God specified. Compare Psalms 87:1: also Mary Magdalene's address to the supposed gardener, speaking of "Him" with whom her heart was full, as if everybody must know that it was Christ whom she meant (John 20:15).
And Israel his dominion - literally, His dominions. The plural expresses excellency.