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.