The book of Acts was written by Luke, and the “former account” he mentions was Luke’s gospel. The former account ends with the day Christ was taken up, which came after He had given commandments or instructions to the apostles, and we will see that a bit later.
Luke also fills in details here that during the period of 40 days, Jesus showed Himself to the apostles and others. When we piece together all that is written about Christ after His resurrection, we find around 10 instances of His appearing to His disciples over 40 days.
Forty days had passed and the resurrected Christ had been right there before them proclaiming the Kingdom of God and all that He fulfilled in the law and the prophets! But they still did not get it. Their hearts may have burned when He was in front of them, but they still were not fully grasping things.
They merely saw Him as the leader of a physical revolution that fell far short of what the Father and Son actually had in mind—the real change God wanted was to turn the whole world upside down (Acts 17:6).
