The reign of God has come in Jesus Christ, but in hiddenness, in humility and lowliness. Jesus came to serve and give his life as a ransom for many (10:45). Jesus promised a triumphant revelation of himself after the cross (14:28), but within Mark (ending at 16:8) the disciples do not yet see the glorious, risen Christ. They only have his Word (16:6–7).
So also we Christians do not see him and the kingdom fully implemented and manifested now. How difficult it is to be faithful on the basis of the Word alone—the disciples are testimony to that fact! If you had been there, it would not have been any easier for you than it is today. The evidence you have is what the disciples and the women had, also on that Easter morning: the promise of His Word, a Word that is ever sure.