Why, from my admittedly limited exposure to western philosopher, do we
place such prominence on arguing for the Christian god's existence or
God is a central concept to all religions. Apart from that, it is an attractive philosophical question. It's been discussed by philosophers even before Christianity. Although I believe it was Abrahamic religions that originated the concept.
Where did the idea that if there is a god, then it must be good come
from, aside from the Bible?
Benevolence is integral to any concept of a Creator. The very fact that God has granted existence to the creation underpins the concept of benevolence. And as for the question of evil in the creation that seems to contradict God's benevolence, I believe Plotinus's theory of evil solves the seeming contradiction.
Plotinus can also be regarded as one of the non-Abrahamic pioneers of the idea of God's benevolence. Though, in his philosophy he uses the term the One to denote the source/creator of the universe and he also equates the One with the Good.
Drawing upon his philosophy, we argue that God (the One) is in essence benevolent because all goodness emanates from Him, whereas evil stems from His creation (i.e. the Intellect, the Soul(s) and ultimately matter as different levels of His emanation). Without evil, there would/could exist no creation. So evil is a necessary part of the universe. In other words, to ask why is there evil, is to ask why is there any creation.
That means the substance of evil is within our very creation. To be liberated from this essential, structural evil, man should redirect his tendency towards material forms (worldly attractions, such as wealth, fame, sexual partners and pleasures thereof) towards intellectual forms (angels) and ultimately the One Himself. However that doesn't imply suppression of our natural tendencies (as practiced by Catholicism) but moderating them so that they don't act as impediment to realization of Intellectual forms and the One (a practice taught by Islam). Plotinus's philosophy, as thus, perfectly substantiates the religious doctrines of God, Original Sin, repentance, heaven and eternal bliss.