"For when the Gentiles, which have not the law, do by nature the things contained in the law, these, having not the law, are a law unto themselves:" (Romans 2:14)
This question crops up at times. Why do we need God to tell us anything? As Paul writes further down the chapter "Which shew the work of the law written in their hearts, their conscience also bearing witness, and their thoughts the mean while accusing or else excusing one another." (2:15) And if you have the above, being the baseline verse for showing how anyone's conscience is a good as "the law", why do we even need God? Because if I'm good without God, my conscience allowing me to get by in this life--a standard of morality, or "good"--why should I worry about answering higher? Good question.
"Behold, I was shapen in iniquity; and in sin did my mother conceive me. Behold, Thou desirest truth in the inward parts: and in the hidden part Thou shalt make me to know wisdom." (Psalm 51:5-6)
So, if you take for granted the premise that there is such a thing as "sin"--defined as the opposite of God's character--and prior to that, the reality of God's existence as elucidated in the Bible, where do you go from there? There's also this theological idea of the atoning death of a living creature to make up for infractions-and-worse. Okay. Admittedly, I'm jumping into this several tiers up but I'm looking to make a point on this level. From "God" to "sin" to "absolution in the person of Christ". These are the key points I feel must be acknowledged before one looks at their life and says "I'm fine on my own". With this statement, it's better one substantiate a life based on a sort-of hopeful nihilism as opposed to clinging to belief in God. If God is real, as He cannot be seen (with our eyeballs), we must reconcile a part of ourselves to the unseen, as well, in order to level the playing field. Because if there are things I'm doing I don't have access to, that could be offending God, maybe I'll look more into my need for understanding and wisdom beyond my limited senses. This is where Christ comes in.
"But of [God] are ye in Christ Jesus, who of God is made unto us wisdom, and righteousness, and sanctification, and redemption. That, according as it is written, He that glorieth, let Him glory in the Lord." (1 Corinthians 1:30-31)
"In the day when God shall judge the secrets of men by Jesus Christ according to my gospel." (2:16)
Begging for questions
One of the things Paul did in writing was unearth the reality of what Christ came to do. Paul calls it "my gospel". Either this is megalomania or else a humble servant doing that with which He's been tasked. One or the other. Christ's mission was to wrest from the natural, physical world, any notion as to our own ability to get right with God. He did everything right. So the question of any one nation or person outside the scope and receipt of the law—is answered. Now that Jesus has come and fulfilled the law, relationship with God has been made possible for any and everyone. Not just a select few.
"For God so loved the world, that He sent His only begotten Son, that whosoever believeth in Him should not perish, but have everlasting life." (John 3:16)