"Wherefore of these men which have companied with us all the time that the Lord Jesus went in and out among us,...Beginning from the baptism of John, unto that same day that He was taken up from us, must one be ordained to be a witness with us of His resurrection" (Acts 1:21-22).
The most important event since the creation itself was the resurrection of Christ, and it was vital that the witness of His chosen apostles focus especially on this great event. They must believe with confidence in His bodily resurrection, having been with Him throughout His ministry, heard His predictions of the resurrection, then seen the infallible proofs thereof, especially the empty tomb and His post-resurrection appearances. Both the original eleven and Matthias, chosen to replace Judas, satisfied these requirements.
Then after the coming of God's Holy Spirit at Pentecost, "with great power gave the apostles witness of the resurrection . . . and great grace was upon them all" (4:33). The resurrection proved that Christ was the Creator and Savior, for only the Creator of life could defeat death.
Paul also saw the risen Christ on the road to Damascus, and thus he also could be an apostle. "Am I not an apostle?...have I not seen Jesus Christ our Lord?" he could say (I Corinthians 9:1). Only those who had seen the risen Lord and been specifically chosen by Him could be true apostles, for they must be credible witnesses of His resurrection.
And that they were! Peter could say: "We are witnesses of all things which He did... whom they slew and hanged on a tree: Him God raised up . . ." (Acts 10:39-40). And Paul could say "God raised Him from the dead: And He was seen many days of them...who are His witnesses" (13:30-31).
Yes, the apostles were true witnesses of Christ's resurrection, and multitudes have received eternal salvation because they were! HMM