Man can be good without God in general, but in the Christian point of view, god's mandate and laws are a pre-requisites. This I espouse for it prevents unwarranted and unreined relativism in morals. Relativism in essence is part of the God-ethics and as Christians we abide by that and guided in our behavior with that. Our knowledge of good and evil is not absolute, neither does man decide absolutely what is good and what evil in all his affairs. For deep in the recesses of man's being, there had been a fore-ordained laws written in his/her hearts and that should contribute greatly tovthe so-called modern-day morals. It is a 'primordial voice' embedded in the inner sanctum of human beings! To contradict it, to disregard it, to obfuscate it....is always a form of violation and likewise disregard of man's integrity as a creature, follower granted by God with such precious gift of discernment, enlightenment and 'spark of one's soul (St. Bonaventure).'
In short, it is ultimately good for the individual and society . After-all once something is justified sociologically and collectively it becomes legitimate for "THAT" society. The philosophical justification in humanism to possess higher ethical obligations to humans, is part of the glue of and for society; and pushes it forward.
It's a necessary function for society, to have that glue, to place that demand upon an individual, for that individual-- to personally strive for and to know this (the above) and to know it in it's essence and practice, and for that individual to use it for societies benefit (in all it's disciplines and functions) therefore being part of the collective. According to Vetlesen, "at the most basic level, receiving direction is what endows the activity one undertakes with its meaning: what makes it carry sense, understood the Hegelian way as sense, not for a separate agent but for all involved."--Surplus of Indeterminacy, Vetlesen.