Humanism would be the opposite of spiritual belief in God, would it not? You probably think this is too simple, but bear with me.
The only exception to this is Christianity, in the sense that humans are made in God's likeness, however faith must be placed in one human, Jesus Christ, and not humanity as a whole. In fact, according to Christianity, the religion of humanism was what formed with the fall of mankind... that is, we no longer put faith in God, but in ourselves.
The humanistic dimension in Christianity is a continuity from Judahism, only Jesus revealed that the radical reciprocity between God and man was not restricted to a single choosen race, the Jews, but opened up to everyone.
I think that the doctrines of original sin and "the fall" are more popular with the opponents of Christianity than they have been historically with Christians, with the possible exception of some of the more radically neurotic sects, none of the Irish monastic or scholarly traditions could have spread the word that sins could be forgiven if it wherent the case or even come up with classifications, original, mortal, veneal etc.
It isnt an insignificant thing, certainly it lead to a lot of confusion and was a major part in the protestant schism and wars that followed in that stead, although I tend to think that books such as Confessions of a Justified Sinner or even Erasmus' discourse with Luther on Free Will provide useful examples that it didnt dominate the minds of all Christians and leave them in a position of either self-loathing or superiority complexes.
The question in one sense comes down to whether God made man or man made God, its very easy to believe presently that man made God, although I think we all have the legacy of the thinking set in train by Descartes, itself a response to studies of refraction, ie that your very eyes could fool you and appearences are not what they seem. Essentially in this model everything starts with the mind and stretches out from there. Although when you think about it that is patently not the case. There are objective natural laws outside of and besides their perception by or rationalisation by the human mind.
The humanism of religions, theist or deist ones anyway, might consider man great etiher because of the reciprocity with God, including either God's communication with the prophets or God's incarnation as a man, Jesus, or because mankind is a pale reflection of angels, the heavenly host or avatars.
A strictly atheist humanism would see that as an impoverishment of man, instead suggesting that whatever is great within theist or deist systems, what is venerated, is infact man's best qualities alienated or seperated from himself and attributed elsewhere.
I think there are also third positions between the two which remain humanistic, that God or the Cosmos is not in any sort of reciprocity with mankind, although mankind has a special role to play because he is conscious and his very existence is a dilemma for mankind.
