The "arrow of God" is a reference to the role Ezeulu plays in this novel. As Chief Priest of the deity Ulu, Ezeulu is in the enviable or unenviable position of being Ulu's messenger, his "arrow."
As Ulu's messenger, Ezeulu is an important part of Umuaro's cultural and religious life, but he is offered little respect and much criticism. (In an interesting plot parallel, Ezeulu doesn't respect the messengers of the white man either.) In the role as God's arrow, Ezeulu is able to punish the village of Umuaro by withholding the announcement of the Feast of the New Yam; without that announcement, the people cannot harvest their new crops, leading to widespread famine.
Ironically, Ezeulu's insistence on punishing the people of Umuaro leads to the end of worship of his deity, Ulu, as people flock to the Christian church for help and protection. So the title Arrow of God might also, ironically, indicate that Ezeulu has become a useful tool in the hands of the Christian God, helping Christianity achieve dominance over Ulu.