On the one hand, Collins says, "I had never really seriously considered the evidence for and against belief." On the other hand, he says, "...if I could no longer rely on the robustness of my atheistic position..." It makes no sense to consider a position never seriously examined to be robust.
Source: The Barefoot Bum
Both can't be true it can't be the case that Collins held an "atheist position" that was "robust" and that Collins "never really seriously considered the evidence for and against belief." A "robust" position is one that is necessarily strong, sound, well-reasoned, multi-faceted, hardy (stands up to many challenges), etc. You can't have a robust position if you've never spent much time seriously examining arguments for and against it.
What's the truth? We may never know and perhaps Collins just isn't introspective enough to know himself that would be one reason why he's unable to provide a consistent account of his own position even over the course of a couple of paragraphs.
Personally, I think it's far more likely the case that he "never really seriously considered the evidence for and against belief." Why? Because I think it's abundantly clear that he still hasn't done so his current rationalizations for his Christian beliefs are so profoundly weak that they don't even qualify as good rationalizations:
Collins turns to Christianity because he finds "no satisfactory explanation in Darwinian evolution" for the presence of moral law. But why do we teach to children precisely those fundamental ethical beliefs that C.S. Lewis (whom Collins references as key to his conversion) uses to support the idea of moral law, especially fairness and the mutual benefit of sharing? And why do we blame the parents, teachers and/or culture, not God, when those beliefs are not properly inculcated?
The inability to find an explanation of moral law in specifically Darwinian (presumably biological, genetic) evolution should not immediately turn one to a supernatural explanation; perhaps the scientific explanation might lie in scientific psychology, sociology, anthropology or some as-yet-undiscovered social science.
What's more, even if the inability to find a naturalistic explanation for some feature of the world could reasonably justify a supernatural explanation, it doesn't necessarily justify "god" as an explanation any other supernatural claim is equally justified given the absence of reliable information about gods or any other alleged "supernatural" entity. Furthermore, Christianity in particular isn't automatically justified by such a situation either another religion like Buddhism, Hinduism, or Islam has just as much to support it. We can hardly consider it a coincidence that Francis Collins just happened to decide upon the religion that is most prevalent where he grew up and where he lives.
However, we could hypothesize that Collins never thought very much about arguments for or against theism and lacked the skills to address the claims and arguments made by apologists around him. Eventually such arguments may have inclined him to adopt Christianity but at the same time he needed to rationalize this in order to maintain an image as a rational, reasonable person so he ends up emphasizing his earlier atheism as if it were a reasoned, considered atheism while describing abysmal arguments as if they were powerful proof for his position.
Now why is it again that anyone should regard Francis Collins as an authority for how and why religion and science can be reconciled? If Collins can't even provide serious, sound, reasonable arguments for believing in a god, how can his arguments on behalf of reconciling belief in that god and science be trusted?