God is often conceived as the Supreme Being and principal object of faith in most religions. He is considered as the creator and sustainer of the universe.
Original series
In the original series, God was not mentioned very frequently. However, he was the one who Mathias Cronqvist blamed for death of his wife, Elisabetha. After Mathias's wife died, he was supposedly bedridden with grief; however, the truth was that he harbored a great resentment toward God. He believed that since he risked his life in the name of God, he did not deserve to have his wife taken from him, especially when the only thing he desired was Elisabetha's safety in return. As a result, he engineered an elaborate scheme to turn into a powerful vampire, claiming that if God's decree was a mortal life, then he should defy it. This was the catalyst for Mathias becoming the vampire the world would later know as Dracula.
In addition, the art of alchemy was described as the principled study of God's creation of this world.
God's power is felt throughout the series, such as in the ending of Castlevania (N64), in which Rosa is resurrected as a human, apparently by divine intervention. It is implied that Dracula being reincarnated as Soma Cruz is a way of giving Mathias a second chance in life.
Lords of Shadow
In Castlevania: Lords of Shadow, God was more prominent. The order to which Gabriel Belmont belonged was essentially an order that protected people in God's name. In addition, the main antagonist Satan's true goal was an attack against God. And even though God never appears, he somewhat acts like an unseen observer.
Throughout the game, Gabriel begins to question his faith, but in the end he sees the truth in God's message in comparison to Satan's goals. He tells Satan that God loves them both and that they only need to ask and they will be forgiven. He rebukes Satan's claim to rule as equals to or above God and for preferring to rule with power and might, rather than offer love and forgiveness. In the end, Gabriel defeats Satan and asks for forgiveness from God.
However, upon learning the truth about the God Mask, and crushed by his failure to revive his wife, Gabriel succumbed to doubts regarding God, especially, upon reuniting with the vampire Laura, questioning how God could allow a child like her, an innocent, to be turned into a creature of the night. His remaining piety and other good qualities are removed once Gabriel allows Laura to turn him into a vampire. Furthermore, upon absorbing the powers of, and defeating, the Forgotten One, Gabriel abandons God, symbolically by crushing his holy weapon, the Vampire Killer, leaving it behind in the dimensional prison that held the gigantic demon.
When Gabriel finally falls utterly into darkness, it later serves to reinforce his belief that God simply isn't worth serving, and that no matter the evils taken down, another would merely take its place as long as evil could flourish in the hearts of men. This existential realization ultimately serves to fuel Gabriel's hatred of God, who allowed such evils to flourish unchecked in the first place and allowed him to suffer just so the greater evils could be undone. Gabriel then resolves to become that evil to counter the other evils while exacting a price for his "protection" by hunting humanity down, becoming a cruel parody of what he once was in order to defy God's will. Despite this, he does not revel in what he has become. God also seems to still love and favor him as his chosen one, as he did not take away Gabriel's immunity to holy power, such as the divine incantation explosion, which wiped out the army of the Brotherhood but left him fully unscathed.
The one and perhaps only act of God seen in the games is the return of the spirit of Marie Belmont, who claims that she was sent back to help Gabriel. One can only guess that it was God who sent her in order to assist Gabriel to fight against Carmilla, as well as aid him in his path of redemption.
Animated series
God does not make an appearance in the animated series; however, the religious institution The Church of Wallachia claims that all their actions are part of his will, including the burning of Lisa Tepes and the attempt to slaughter the Speakers. Later, the Bishop responsible for Lisa's death claimed that his lifework was in God's name, but is faced with the claim that God has abandoned him, for his despicable fanaticism and crimes make him sick. Indeed, the Church the Bishop belongs to provides no sanctity and does not protect him from Blue Fangs, implying that God considers the bishop's gory fate well deserved.
Later, God's power manifests when a pure-of-heart priest channels it to make Holy Water, then used to turn the tide against Dracula's armies of demons.