This is an extension to the theories about squires found in this thread: http://www.17thshard.com/forum/forum/54-words-of-radiance-discussion/
From the epigraph of chapter 54 of Words of Radiance, we have:
This quote, combined with a quote from Dalinar’s vision at the Purelake,
heavily implies that loyal followers of a Knight Radiant can gain access to powers granted by Stormlight, and that the Windrunners are one of the Orders to have such squires. At the end of Words of Radiance, Lopen is able to absorb Stormlight and use it to heal his arm. I think it is pretty obvious that that this indicates that he is the first of Bridge Four to become a true squire. But that leaves the question of why he is the first. There is nothing to show that Lopen is any more loyal to Kaladin than any of the others in Bridge Four.
This leads me to reconsider the way the Nahel bond and its corresponding Oaths/Ideals work. We know from WoB that Perception and Intent are very important to Investiture in all of the books in the Cosmere:
So this is the meat of my theory: The Nahel bond is not about oaths, but Intent. The oaths, except for possibly the first Ideal, are not actually concretely defined, but representations of the Intent of each Herald. Each Herald/Order has a purpose but that purpose doesn’t necessarily have to be defined, but some Orders have additional Ideals defined to provide an explicit outline or reminder of their purpose. This is further supported by another section of the WoB:
This is why at the end of Words of Radiance, Pattern tells Shallan that as a Lightweaver, she only has one Ideal; this possibly means that the Lightweavers (and maybe even Shallash herself) chose not to define their purpose in such a concrete way. Pattern implies that she has said all of the words that she is required to say, but still has truths (i.e. some aspects of the the full Intent of the Lightweavers) to fully discover.
This is why some spren are attracted to humans even before they even begin to say the Ideals. Sylphrena and Pattern both were attracted to Kaladin and Shallan because they demonstrated the true Intent of their respective Orders. Though we don’t know exactly what the Intent of the Lightweavers is yet, we do know the Intent of the Windrunners.
Kaladin’s progression through the Ideals of the Windrunners is very strong evidence for this theory. At the end of TWoK, he discovers and speaks the Second Ideal when acting upon the Intent that the Second Ideal was meant to describe. The same thing happens at the end of WoR when Kaladin acts upon the Intent of the Third Ideal. It is important to note that the Intent comes first, which then triggers the “discovery” of the Ideal. I believe that it isn’t the speaking of the Ideals that is necessary to become a full Radiant, but believing them and fulfilling them through action.
Now back to Lopen and the squires. Why just Lopen and not all of Bridge Four? Not only is Lopen loyal to Kaladin, but he has acted on the Intent of the second Ideal by protecting Elhokar. There is a bit of an assumption here because we don’t see what is going on in Lopen’s head, but I believe he has truly committed himself in protecting the King, and that is how he gained the ability to use Stormlight. The reason the other members of Bridge Four haven’t gained this power yet is because they haven’t committed themselves to the Intent of the Windrunners, and are still acting purely out of loyalty to Kaladin.