Quite huge actually, but maybe not that crucial.
The most important changes would be
- Sexuality : While Augustine was more of a puritain in his earlier days, he evolved (for M.Rouche) from this position to a more comprehensive way, agreeing that the couple pleasure was an essential part of the Christian life and not something "foreign". Without him, Church may continues a more roman point of view on matrimonial union, leading to less religious pressure to stabilize marriages eventually.
Concubinage and semi-polygamy may be the rule for a longer time, the intellectual/theologian reject of sexual pleasure within the marriage as well.
- Philosophy : Augustine managed to link Greco-Roman intellectual legacy, especially a form of neo-platonism, with the Church which may have a less coherent, solid view of a transcendant God, above man and its realisations but not foreign to the human as individual. Augustine was one of the first of his era to allow the reinforcement of the *ego* in an ethical way, where the man is allowed to approach a God that he can't fully understand but that can fully understand him.
Finally he was one of the architects of the adaptation of Christianism to its era, without compromising its principles. I'm not saying that without him, it would fail, but you would have probably more theological disputes, debates, and the process could go longer.