Can Christians be humanists and moralists?

[Mudhammam]

The only other context for your question is that there is some form of universal law of morality, which would then have to be defined, which you have not done, thereby making the question unanswerable.

Morality is universal in the sense that there are objective conditions (i.e. the world) upon which human beings can increase their sense of happiness, comfort, pleasure, etc., or vice versa, enhance their misery, suffering, decrease their freedom, etc. Of course, particularities will differ but that's where discussions within a framework of rational humanism, along with experience, can allow for a better morality to emerge, as opposed to an absolutist's dogma declared via revelation. Generally when, for example, we see someone stabbing themselves in the name of "pleasure," we agree that they're not a person who is acting rational, so if you want to appeal to special cases such as that, I don't think you'll find any problems in this discussion that are exclusive to moral issues.

inherent knowledge of what is right or wrong.

Based on a rationale that puts the well-being of persons first or some rash ideal dictated by an imagined being?

I admit that I have difficulty in seeing the sense of this question, as it is entirely based upon morals, which you cannot judge people upon (let alone whole groups) unless based upon a universal moral foundation, something that I don't see can exist from the viewpoint of an anti-theist, as what is right or wrong is surely by your logic, a matter of opinion.

Once you agree that it's better to exist in a state of freedom, security, and happiness, as opposed to torture, oppression, and fear, then it's really quite sensible to talk about morality in terms of an objective logic in the same manner that one speaks of arithmetic. Of course, if you deny the first premise, and declare your enjoyment for self-mutilation, it's fair to say you'll probably find as many allies as you would if you were to insist that twice two equals five.