Only God is holy by his very essence; however, by a person, place, or thing’s association with God, it too can be called holy. To be called holy is to express the idea of consecration, that someone or something belongs to God. That is why the Bible can call many persons, places, and things holy.
In Genesis 28:16, the place God appears is "holy." In Exodus 19:6, God tells the Israelites through Moses, "and you shall be to me a kingdom of priests and a holy nation." God’s dwelling place in the Tabernacle is "holy" (Ex 28:43), as is the city of Jerusalem (Is 48:2). Even a goat, the victim of sacrifice to God, is called "holy" in Leviticus 10:17.
After Christ’s death and resurrection the Christians called themselves and each other "holy ones" or "saints," called by God to be his (Rom 1:7). In 1 Peter 1:16 we read, "it is written, ‘You shall be holy, for I am holy.’"
Since we are his holy people, and his people are the Church, it is fitting that the head of his holy people be called Holy Father—not because of his own merit, but because Christ died for him and for the Church that he leads on earth.