I don't own Supernatural.
The Strength of a Soul
When asked, most people would describe Sam Winchester as fragile and sweet. Even when he was addicted to demon blood and even when he was soulless, people felt compelled to protect him, especially his brother. Dean was worse than most, though. Since the night of the fire that covered the demon style killing of his mother, he had protected Sam at all costs. It was this protective side that caused him to go to Hell for the life of his little brother.
John believed his youngest was delicate. He knew Sam could hunt the supernatural but he couldn't understand why his youngest didn't want to. Sam was always looking for the good in others and wanted a normal life. In John's book, that made Sam weak. He knew that Dean would defend Sam when Sam was being picked on. Sam, for the most part, refused to fight unless absolutely necessary.
Even some Angels and Demons felt the same way. Not to protect Sam but to use his seemingly overabundance of softness and trust. Ruby used his feelings to kill Lilith and thus assuring that Lucifer would rise. Crowley used it when he wanted to keep Lucifer from winning the fight with Michael and then when he wanted the Alphas. And the list goes on.
The only ones who didn't use Sam's feelings were Bobby, Pastor Jim, and Gabriel. They were very much aware of the thin spun shaft of steel that ran through Sam's soul. Each in their own way tried to temper that steel into a sharply honed weapon. One that only cut evil and would serve the purpose of God.
Bobby had been a barrier between Sam and his father whenever they were at his place. John had a tendency to be harder on Sam than his brother since Sam seemed to be delicate. Bobby also taught Sam most of the lore and legends that were in his books. Sam could also read and write at least five dead languages and most demonic and angelic sigils. He gave Sam the strength of knowledge and loyalty.
Pastor Jim taught Sam about faith and hope. He showed Sam that God was real and did listen to prayers, though He may not answer them the way the supplicant would like. He showed Sam that hope was ever present even if you couldn't see a way out. John had neglected that particular lesson for his sons. He had lost his faith when Mary was killed. Jim also taught Sam about forgiveness and atonement. He told Sam that if Sam asked God for forgiveness, he would be forgiven. Jim gave Sam spiritual strength.
Gabriel gave Sam the knowledge of family and how to stand alone. Gabriel had been attracted to the strength of Sam's soul and the childlike purity of it. Nothing else could've brought him out of his self-imposed exile. It was through his "lessons" and small tests that he found that Sam was a warrior who would be able to do the impossible. The last lesson for Sam was when Gabriel died standing up to his brother for the sake of others. Gabriel gave Sam the strength of self-sacrifice and the love of billions of lives that Sam would never meet.
Because of these gifts, Sam was able to say yes to Lucifer and eventually take back control of his body. It was because of his strengths, he was able to open the Box and throw himself, Lucifer, and Michael into it. The gift of atonement made it possible for him to stay in the Box and work through his guilt, even when Castiel came to get him. Sam's strength allowed his soul to stay even when Cas was pulling his body out.
Only Death was stronger that Sam. Death found his soul hiding behind his torturers, trying to stay where he felt he belonged. Even when Death finally got hold of Sam, he still struggled to stay. Death was surprised at the strength he felt and was strangely humbled. It was only when Death told Sam that Dean needed him that Sam stopped struggling and went willingly.
Sam finally showed his true strength when Cas pulled the wall down. He was able to rise off the cot to try to rescue Dean and Bobby from the insane angel. He endured the hallucinations of Lucifer quietly so his brother and adopted father wouldn't worry. He was the only one who reached out to Cas with forgiveness and a plan to save his friend. It was through Sam, that Cas was saved.
Sam was the strong one when Bobby died. Dean had been broken by the death and couldn't pick up the pieces until Sam got him going against the Leviathans. Dean didn't want to believe that his fragile little brother healed him. What Sam didn't know was that Cas and Gabriel both gave him a gift of part of their Grace. The graces resided in his soul along with his strengths and gave him a power he wasn't aware of, the gift of healing.
Sam would never be known as the person who sacrificed himself unless you read Chuck's books. He would never be known as a warrior or a healer. Many people would see him as weak or delicate but they would be dead wrong. Sam was the strongest person in the world.
Thank you Diayu Amaya for beta-reading.