@ David Stratton, in reference to: "This answer would be a lot better if you could add references showing that this is a common understanding, and who teaches/believes it. On this site, we're not looking for personal interpretation, but rather focusing on what various Christian groups teach. See How we are different than other sites? and What makes a good supported answer?
David, is not one a whole? Are not "groups" sets of one; like "one" group. Can a group be formed without the first "one?" Whether one idea, or anything else that goes on to form a group? Did not Jesus begin as just himself, just a "one?"
Also, the answer is definitely relative to sin. What is sin? The original question was; "What is the basis for saying that Adam was created without sin?" Therefore, the definition and true meaning of what constitutes "sin" must be established first, before asserting whether or not Adam was created without sin. Additionally, If I'm not mistaken, the true intent of this blog is not citing passages of the Bible, or to ascertain any information as to; authors, dates, originality, translations, or any anything relating to scriptures as to assess authenticity, or relation of current literature to original writings, so as to form a correct interpretation, since there are so many different interpretations. But God's words (instructions) to man have to be firm, unchanging, clear and to the point, no matter how many times rewritten, translated, and interpreted. It is ludicrous, and unholy to think that God's words and true intent could be "lost in translation." Is it not the intent of this site, and the blogging therein, for the purpose of getting at the truth? Therefore no "one" stone shall remain unturned, no matter how small, or unlikely it is retained to be hiding the truth.
Now to the original question. Is it not obvious that God wants mankind to learn about good and evil through experience and making mistakes along the way? Is not experience the best teacher? Well, God is the best at everything, including teaching. God could have made mankind fully wise to begin with, but in his superlative and infinite wisdom, or maybe just for his amusement, elected "other-wise", (no pun intended). But why cannot even the simplest minds understand what is "free will?" What is justice? God is Just, God is Good. Would God, in his Essence of perfect justice and goodness, allow free will to mankind when mankind had no knowledge of good and evil? How can a man exercise free will with this knowledge and not risk getting in trouble (sin)? How can man choose wisely and correctly? Would man just flip a coin; head is good, tail is bad? We are all children of God, bar none. Ask Jesus. Did he not teach how to pray correctly? Did not Jesus, in leading others to pray, begin by saying; "Our Father" Note that it was not "My Father" nor ""God" who art in Heaven.""
God is the Father of all and creator of everything.
God is the Greatest playwright, and He wrote this great drama of ours, and which is a comedy, a satyr play, and a tragedy, all in one. So just sit back, relax, and be entertained; the Author will come through the curtains, and make his stage appearance at the end. And along with accepting all the clamorous, long, and appreciative applauds, He will show His "humble" appreciation by bowing to the crowd.