"Gandalf is an angel," Tolkien wrote in a letter to a fan long ago, and it was decades before we could learn more fully what he meant. First of all, Gandalf is a messenger or emissary, the precise translation of the Hebrew mal'akh or Latin angelus. But Gandalf is also a Maiar, a lesser angelic being in the parlance of the legendarium, and is a spirit incarnated in a form that can feel weariness, hunger, fear, and pain, but supported by the angelic nature within to endure long and only slowly show signs of age. He is sent with at least five other "wizards" from the West by the Valar, superior demiurgic angelic beings, as a counter to the growing threat of Sauron in Middle-Earth, not to oppose him by acts of power (which in the past have caused vast cataclysms and destruction to the fabric of the world), but to train, advise, instruct, and encourage the peoples of Middle-Earth to rise up and resist Sauron themselves. As such they appear as old but wise men, not as heroes or figures of awe.
Many point out the similarities of Gandalf to Odin or Merlin or the Mountain Spirit; fewer mention his resemblance to figures in the Bible, and then it is mostly to compare his resurrection to that of Jesus. But he also seems to owe something to the prophets and judges of the Old Testament, especially Moses, Elijah, and Samuel. He has a wonder-working staff, like Moses; a mantle, like Elijah; and he crowns the new king like Samuel anoints both Saul and David. His influence is mainly moral as he travels through the world, giving advice, encouragement, and exhortation; he confounds rulers like Elijah did Ahab or Nathan did David, and heals a king like Isaiah did Hezekiah. And it is a curious fact (to go outside the Bible for a moment) that the investiture of both Gandalf and a bishop of the Catholic Church includes a pointy hat, a staff, and a ring. So I believe there is a subtle but definite Christian strain behind the image of Gandalf.
The Hildebrandt picture seems a fairly strong example of this; it could be titled "The Transfiguration of Gandalf," referring to the moment Jesus reveals his glory to the disciples on the Mount of Olives. The Douglas Beekman Gandalf afterwards makes the religious parallel fairly obvious by giving the wizard a staff that is plainly a crosier, and the Peter Cabas Gandalf after that is from a picture of the Council of Elrond done in a Renaissance style and posed to resemble the Last Supper. In the last picture, by Inger Edelfeldt, Gandalf crowns Aragorn king, giving him the heavenly seal of approval: "Now come the days of the King, and may they be blessed while the thrones of the Valar endure!"