After spending three years with the Lord, the apostles still struggle spiritually (Matthew 16:23; 26:34; Mark 10:35-41; Luke 9:54-55). Even as they see the risen Lord and receive the Great Commission, some are still doubting (Matthew 28:16-20).
Afterward [Jesus] appeared to the eleven themselves as they were reclining at table, and He rebuked them for their unbelief and hardness of heart, because they had not believed those who saw Him after He had risen. And He said to them, “Go into all the world and proclaim the Gospel to the whole creation.” (Mark 16:14-15).
Jesus is so powerful that He can work through frail, flawed people. Empowered by the Holy Spirit, the apostles gain courage. They grow more like Jesus and become effective evangelists. Yet, still, they do not look impressive to worldly eyes.
Now when [the authorities] saw the boldness of Peter and John, and perceived that they were uneducated, common men, they were astonished. And they recognized that they had been with Jesus (Acts 4:13).