When Jesus walked on the earth in human form, He plainly told His disciples that He would depart and then return for them one day. When He did return, He would take them back to heaven with Him (what modern Christians refer to as “the Rapture”). For example, on the night before His crucifixion, Jesus said to His eleven faithful apostles:
Let not your heart be troubled; believe in God, believe also in Me. In My Father’s house are many dwelling places; if it were not so, I would have told you; for I go to prepare a place for you. And if I go and prepare a place for you, I will come again, and receive you to Myself; that where I am, there you may be also (John 14:1-3, emphasis added).
Clearly implied by Jesus’ words was the possibility of His return during the lifetimes of the eleven. In fact, after hearing what Jesus said, they would have simply assumed that He would be returning for them within their lifetimes.
Jesus also repeatedly warned His disciples to be ready for His return, again implying the possibility of His return within their lifetimes (see, for example, Matt. 24:42-44).