One of the qualifications to being an apostle is to have seen the resurrected Christ. Hence they are witnesses to his resurrection. This is similar to the doctrine of the trinity in that the word is not found in the bible but you are forced to come to conclusions based off of a number of verses.
The apostle Paul writes that he has seen the resurrected Christ:
1 Corinthians 9:1 - 1Am I not an apostle? am I not free? have I not seen Jesus Christ our Lord? are not ye my work in the Lord? 2If I be not an apostle unto others, yet doubtless I am to you: for the seal of mine apostleship are ye in the Lord.
The apostle John writes that he has seen Christ (both pre and post resurrection). Also who are the "we" and who are the "ye/you" in the following passages?:
1 John 1:1-4 - 1That which was from the beginning, which we have heard, which we have seen with our eyes, which we have looked upon, and our hands have handled, of the Word of life; 2(For the life was manifested, and we have seen it, and bear witness, and shew unto you that eternal life, which was with the Father, and was manifested unto us;) 3That which we have seen and heard declare we unto you, that ye also may have fellowship with us: and truly our fellowship is with the Father, and with his Son Jesus Christ. 4And these things write we unto you, that your joy may be full.
The apostle Peter writes to all believers reading his epistle (that includes everyone alive today or else who are the "ye" in the following verses?) that they have not seen Jesus Christ:
7That the trial of your faith, being much more precious than of gold that perisheth, though it be tried with fire, might be found unto praise and honour and glory at the appearing of Jesus Christ: 8Whom having not seen, ye love; in whom, though now ye see him not, yet believing, ye rejoice with joy unspeakable and full of glory:
