Thus it is written, and thus it behooved the Mashiach (the Annointed One) to suffer, and to rise from the dead the third day; and that repentance and remission of sins should be preached in His Name among ALL nations, beginning at Jerusalem.  And ye are witnesses of these things.    Luke 24:44-49 (KJV)

Question:  Did Yahshua's disciples fully understand what was about to occur regarding the crucifixion, resurrection & ascension? No, Yahshua opened their understanding regarding what had been prophesied about Him. 

Yahshua did not come to destroy the Law: 

Think not that I am come to destroy the Law, or the Prophets; I am not come to destroy, but to fulfill. Verily I say unto you, till heaven and earth pass, one jot or one tittle shall in no wise pass from the Law, till all is fulfilled. Whosoever therefore shall break one of these least commandments, and shall teach men so, he shall be called the least in the Kingdom of Heaven, but whosoever shall do and teach them, the same shall be called Great in the Kingdom of Heaven. Matthew 5:17-19 (KJV)

All things must be fulfilled, which were written in the Law of Moses, and in the Prophets, and in the Psalms, concerning me.   Luke 24:44  (KJV)

Read & Study Matthew 5:17-18, after which, do the same with the whole chapter; be very attentive to what Yahshua said.  The disciples believed that Yahshua was the Son of the One True and Living GOD, and believed that He sent His Son into the world (John 3:16). After Yahshua's crucifixion, resurrection and ascension, Yahshua's disciples waited in Jerusalem (as Yahshua instructed them Acts 1:3-5) and believed and received the promise of the Holy Spirit as was prophesied within the book of Joel chapter 2!