Who decides who is qualified and where does that authority to decide come from? Where did it originate?
Jesus wasn't there after the crucifixion. How did the apostles remember His teachings? John 14:26 tells us that the Holy Spirit gave remembrance. When they were preaching and writing their books, where did the inspiration come from? The book of Acts tells us that the apostles received directly from Jesus through the Holy Spirit (Acts of the Apostles 1:1-2, Acts of the Apostles 2:4).
Jesus Himself promises to give the Holy Spirit to anyone who asks.
Luke 11:9-13 And I say unto you, Ask, and it shall be given you; seek, and ye shall find; knock, and it shall be opened unto you. For every one that asketh receiveth; and he that seeketh findeth; and to him that knocketh it shall be opened. If a son shall ask bread of any of you that is a father, will he give him a stone? or if he ask a fish, will he for a fish give him a serpent? Or if he shall ask an egg, will he offer him a scorpion? If ye then, being evil, know how to give good gifts unto your children: how much more shall your heavenly Father give the Holy Spirit to them that ask him?
If we are a royal priesthood, each of us must be given discernment.
1 Peter 2:9-10 But ye are a chosen generation, a royal priesthood, an holy nation, a peculiar people; that ye should shew forth the praises of him who hath called you out of darkness into his marvellous light; Which in time past were not a people, but are now the people of God: which had not obtained mercy, but now have obtained mercy.