Throughout the Harry Potter book and movies series, it was clear that Dumbledore knew Harry would survive Voldemort's attack in Harry Potter and the Deathly Hallows. With Harry being able to come back to life, he was able to defeat Voldemort and save the wizarding world. To ensure that the final battle would end in Voldemort's end and also Harry's survival, Dumbledore put in place certain measures.
From the time up until Dumbledore's death in the sixth book and film, it was known that besides being one of the cleverest and whimsical wizards in the wizarding world, Dumbledore was also one of the most secretive. While performing his duties as Headmaster and leader of the Order of the Phoenix, Dumbledore would have backup plans put in place in case his original plans would fail. This included having Snape act as a double agent and then planning his own death so that Draco would not be the one to kill him. Moreover, Dumbledore was able to gather useful information which would serve to help him with his planning.
Due to his strategic mind, careful planning, and skill at gathering information, Dumbledore seemingly knew that the love protection spell that Lily placed on her son at the time of her death would allow Harry to survive the Killing Curse for a second time. As a result, the protection spell would continue to be in effect until the final battle between Harry and Voldemort. Furthermore, Dumbledore guaranteed that Harry would become the fabled Master of Death through possessing the three Deathly Hallows.
During the Harry Potter series, the power of love was a constant theme and motif, most particularly a mother’s love for her child. This was explicitly illustrated by Lily’s love for Harry. After Harry’s first battle with Voldemort in Harry Potter and the Philosopher’s Stone, Dumbledore mentioned to him that the reason he was untouched was because Lily "sacrificed herself for you, and that act leaves a mark". This selfless act of love would serve to protect Harry numerous times, even up to the point Harry’s blood was used to bring back Voldemort in The Goblet of Fire. Though it was seen that Voldemort was able to touch Harry, the books further explain the significance of this act, and how Dumbledore knew that this would help Harry. At the end of the fourth book when Harry is explaining to Dumbledore what happened, it stated that "there was a gleam of something like triumph in Dumbledore’s eyes", implicating that Dumbledore knew of the consequences of Voldemort using Harry’s blood. This is confirmed later on when Dumbledore told Harry in the seventh book: “His body keeps her sacrifice alive, and while that enchantment survives, so do you."
Moreover, Dumbledore knew that Harry would survive by making sure that he would possess the three Deathly Hallows, which would make Harry the Master of Death, as suggested by a fan theory (via Reddit). Mentioned in The Tales of Beedle the Bard, anyone could become this Master if they possessed the Invisibility Cloak, the Elder Wand and the Resurrection Stone. As a young man, Dumbledore was fascinated by the tale and was believed that gaining all three Hallows would make someone invincible. In The Philosopher’s Stone, Dumbledore gifted Harry the Cloak, and later on, bequeathed Harry the Stone. Harry then became the true owner of the Elder Wand after disarming Draco, thus becoming the Master of Death. As a result, not only was Harry able to save the wizarding world, but he also overcame death.
Dumbledore was a master strategist and tactician. Through his faith in love and profound knowledge in tales, Dumbledore was able to foresee that Harry would be able to survive Voldemort during the final battle in Harry Potter and the Deathly Hallows. In addition, Dumbledore had faith that Harry would succeed in bringing peace to the wizarding world.
