When Christ visited the Nephites, He gave the twelve disciples power to bestow the gift of the Holy Ghost on others (see 3 Nephi 18:37). Thanks to Moroni, we have the actual words He spoke to them as He bestowed this priesthood power. Notice that He told them they had to pray mightily before they could begin using the power He’d given them. It’s a little like receiving a new debit card in the mail. It is valid for transacting your bank business when you receive it, but before you do, you have to activate the card by calling a number on a little sticker on the card. Once activated, you’re free to use the card. The Nephite disciples were free to use the priesthood power Christ had given them once they had “activated it” through mighty prayer.