The disciples were the ones who believed in the baptism and blood of Jesus as the pivotal elements of their salvation. Why? The reason is well described in the passage from Acts 1:21-26. 

“Therefore, of these men who have accompanied us all the time that the Lord Jesus went in and out among us, beginning from the baptism of John to that day when He was taken up from us, one of these must become a witness with us of His resurrection.”

When the Apostles selected another Apostle to replace Judas, they selected someone who believed in and from the point that Jesus’ received His baptism through which He took upon all the sins of mankind. In other words, having faith in Jesus’ baptism was the most important qualification for the disciples to be the supplementary Apostle.

The disciples of Jesus believed in His whole public ministries: His Godhead, His baptism, crucifixion and resurrection. Like this, to become an Apostle, one had to believe that Jesus had received baptism from John the Baptist to take the sins of the world onto His body.

To become His disciples in this last age, we, too, have to believe according to the disciples faith. If people ignore any single element of Jesus public ministries—His being God, His baptism through which He took upon the iniquities of sinners (Matthew 3:15), His bloodshed on the Cross or resurrection, then they cannot become His disciples.