The Call of the 12

“And he ordained twelve, that they should be with him, and that he might send them forth to preach.” (Mark:3:14And he ordained twelve, that they should be with him, and that he might send them forth to preach,
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Early in His public ministry, Jesus gathered around Himself those to whom He would eventually entrust the Christian message. Many others had also been attracted to Him and His works, as indicated in the previous verse: “And he goeth up into a mountain, and calleth unto him whom he would [emphasis in the Greek is on “he”; the choice was His alone]: and they came unto him” (v. 13). Of those He invited, He “ordained twelve.”

Such a momentous selection could not be taken lightly, and we should not pass over it either. Luke gives us further information: “He went out into a mountain to pray, and continued all night in prayer to God” (Luke:6:12And it came to pass in those days, that he went out into a mountain to pray, and continued all night in prayer to God.
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Four purposes are listed for these appointees, but the last three flow from the first: “That they should be with him.” They would see Him in action, learn truth from Him, assist Him in His work; but most importantly they would see His character and habits, and would never be the same.

Part of their training included being sent out to put in practice what they had learned, “that he might send them forth to preach, and to have power to heal sicknesses, and to cast out devils” (Mark:3:14-15 [14] And he ordained twelve, that they should be with him, and that he might send them forth to preach, [15] And to have power to heal sicknesses, and to cast out devils:
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A study of these disciples as revealed in the gospels makes one wonder if Jesus made a proper choice. However, in the book of Acts, once He was gone and the Holy Spirit empowered them, we recognize that their training was complete. We are the result of their effective ministry. 

https://www.icr.org/article/call-twelve/