Resurrection is an act of God, wherein human beings achieve the fullness of life intended by God, when He created Adam but lost it through his sin and death.
God proved that He had the power to resurrect, when the dead branch of Aaron budded almonds, to prove that only Aaron and team were called and not Korah's team (Numbers 17:8). Elijah and Elisha raised the dead in the Old Testament and Jesus had raised many from the dead during His ministry.
However, before Jesus died, His disciples could not understand or believe when He taught about His resurrection. In spite of Jesus teaching many times about the resurrection, the concept of coming back to life did not sink into their minds.
Matthew 22:23-28 – The Sadducees, who say that there is no resurrection came to Jesus and asked Him a question saying, “Teacher, Moses said, ‘If a man dies, having no children, his brother must marry the widow, and raise up children for his brother.’ Now there were seven brothers among us; the first married, and died, and having no children left his wife to his brother. So too the second and third, down to the seventh. After them all, the woman died. In the resurrection, therefore, to which of the seven will she be wife? For they all had her.”