We must be worthy of the Spirit. When we speak by the Spirit, the Spirit will carry our message into the hearts of those whom we teach. Every missionary knows of this commandment and promise of the Lord:
Therefore, verily I say unto you, lift up your voices unto this people; speak the thoughts that I shall put into your hearts, and you shall not be confounded before men;
For it shall be given you in the very hour, yea, in the very moment, what ye shall say.
But a commandment I give unto you, that ye shall declare whatsoever thing ye declare in my name, in solemnity of heart, in the spirit of meekness, in all things.
And I give unto you this promise, that inasmuch as ye do this the Holy Ghost shall be shed forth in bearing record unto all things whatsoever ye shall say. (D&C 100:5–8; compare Romans 5:5.)