D&C 21

From the beginning of the organization of the Church, the importance of priesthood keys held by the Prophet becomes very apparent as the the Lord said -

"Wherefore, meaning the church, thou shalt give heed unto all his words and commandments which he shall give unto you as he receiveth them, walking in all holiness before me;
5 For his word ye shall receive, as if from mine own mouth, in all patience and faith." (Doctrine and Covenants Section 21:4 - 5)

And with that commandment, the Lord provides this promise -

"For by doing these things the gates of hell shall not prevail against you; yea, and the Lord God will disperse the powers of darkness from before you, and cause the heavens to shake for your good, and his name's glory." (Doctrine and Covenants Section 21:6)

I believe that is power and authority of priesthood keys dispensed in this dispensation. The Lord continues with this promise also -

"...I will bless all those who labor in my vineyard with a mighty blessing, and they shall believe on his words, which are given him through me by the Comforter, which manifesteth that Jesus was crucified by sinful men for the sins of the world, yea, for the remission of sins unto the contrite heart." (Doctrine and Covenants Section 21:9)