Can Atheist be objectively moral in the case of a possible follower of an infallible?

[Mystic]

A very simple and common definition of Good is that is which assists your fellow man. Sometimes this can sit perfectly comfortably alongside religious "revealed" morality; taking care of the sick, feeding the hungry and other good works. Where it really differs is in what I would call "arbitrary" morality, actions which have no effect upon your fellow humans (thus neither good nor bad according to the definition I listed) but are singled out as horrifically evil by religious morality. This ranges from everything from swearing in private to killing everything that doesn't agree with you.

It's very easy to have notions of good or bad without a deity, we're pack animals by nature and like all pack animals we have an interest in the survival, growth and continuation of the pack. 

Sure, we can have our own definition. I am not disagreeing. It's just while we use the same word, the reality of them both are completely different and they have no comparison.

But supposing your premise of the spirit guide is correct, what guarantee do we have that this force wants to help us or lead us toward "good"? What is to say the higher power isn't entirely apathetic or actually evil? What's to say the higher power you describe is actually Iblis trying to deceive everyone? Even if such a force revealed itself, why should we assume following it is the right course of action?

This is a good question. First, the reality of the divine names of God is such that one of them is God is Truthful, trustworthy. The Quran similarly describes the word of his light, the Prophet "One to be obeyed, one to be trusted". In other words, if you see the light of the holy names of God in the light of this soul, and it's pure light, and pure beauty, you can see it is trustworthy and you can witness it is "true life".
This is while Iblis or devils in the unseen are "dark" and "unclean". They can't take on the clothes of honor of God and bestow it upon believers, they can't manifest glorification of God sincerely, they are far removed from God's valley, they can't show inward spirit of exalted honor, they are rather cursed and far removed from God.
Therefore if you ever witness such guides, they aren't deceives, and you can see the trustworthiness manifest in their light spiritually and what they are manifesting to you to imitate you see the praise and beauty to imitate inwardly and spiritually.
So the question of deception is impossible.
Of course you need such a guide if God exists and there is many stages of to traverse, and praiseworthy exalted stations to reach.