The medieval Christian view was highly sympathetic to risk capital formation. The key was the word "risk". What was objectionable to the medieval mind (and Mohammed confirmed that this was far from exclusively a Christian view) was the receipt of a return independent of the success or otherwise of the enterprise being financed. This was seen as exploitative and an offence against charity.
Aquinas elaborated this by arguing that, unlike, say, land, which you could own and farm yourself or own and let somebody else farm for a rent, the use of money was inseparable from its ownership. Money had no inherent value; it was merely a token. To charge interest was therefore to charge for something that did not and could not exist - the use of money separated from its possession.
Medieval society had a burgeoning commercial class. "Creative accountants" were regularly coming up with schemes for making interest look like something else and Church courts were constantly rejecting them, with the same mixture of weariness and zeal that Gordon Brown brings to his battle against tax avoidance.