The issue was the Resurrection, and still is

And with great power gave the apostles witness of the resurrection of the LORD Jesus: and great grace was upon them all. Acts 4:33

Witness of the resurrection of the LORD Jesus. This has two aspects, i believe. The apostles testified that they had witnessed the bodily resurrection of the LORD Jesus. And this testimony came with great power and great grace; not in word only, but with a manifestation of power : God bearing witness thru them that they were telling the truth, and indeed the power manifest, thru various phenomena, was credited by many as being the work of the resurrected Messiah in the midst of His people. Their testimony being proof, and, Jesus Himself being manifested in power, proof that He is risen and in the midst of His people. Proof that He is present to SAVE the lost. Proof that He is present to keep His people, and to judge all unrighteousness.