I am intrigued about what people think of souls. Most often, they'll think souls as a separate entity from the body, and that the soul is what defines you and is uniquely given to you by the powers that be. If that were so, why do mere bodily afflictions cause you to change personalities, as if changing souls.
And to answer your question, aside from our instincts to survive, to reproduce, and make sure your genes remain in the gene pool (which I know you don't want to hear), you are right in saying we have no purpose, but you fail to recognize that we have free will, regardless of whether or not if there is a supreme being. The will to persevere, and hope for better lives, things that aren't exclusive to religious people.
Sure, life at times will seem rather hopeless, sorrowing, or purposeless, but we have a choice to end it or to continue on. After all, man created religions for as long as we gained sapience, and mankind giving explanations on the whats, the hows, the whens, the wheres, and the whys surrounding life, purpose, survival, and existence as a whole, and with the earliest known religion dating back to around ~7,000 years, who's to say we cannot create our own purpose, give ourselves hopes of our own?