Objective truth for the basis of morality that is compatible with atheism or theism:
From the
Kalama sutta:
What do you think, Kalamas? Does greed (or hatred, or delusion) appear in a man for his benefit or harm?" — "For his harm, venerable sir." — "Kalamas, being given to greed, and being overwhelmed and vanquished mentally by greed, (or hatred, or delusion) this man takes life, steals, commits adultery, and tells lies; he prompts another too, to do likewise. Will that be long for his harm and ill?" — "Yes, venerable sir."
<...> (Same passage substitute hatred or delusion for greed)
"What do you think, Kalamas? Are these things good or bad?" — "Bad, venerable sir" — "Blamable or not blamable?" — "Blamable, venerable sir." — "Censured or praised by the wise?" — "Censured, venerable sir." — "Undertaken and observed, do these things lead to harm and ill, or not? Or how does it strike you?" — "Undertaken and observed, these things lead to harm and ill. Thus it strikes us here."
The sutta goes on to say that doctrines preaching, greed, hatred, or delusion lead to long term harm, no matter how much pretzel logic they may try to apply to it to justify it--so reject those that preach thusly. Doctrines that preach a
lack or ending of of greed, hatred, or delusion lead to long term benefit, so accept them.
If your mind is overtaken by such things to cause you to commit harmful ill deeds, then that is an objective truth and an objective basis for morality, and requires no god.