We are not saved to be "channels only," but to be sons and daughters of God. We are not turned into spiritual mediums, but into spiritual messengers; the message must be part of ourselves. The Son of God was His own message, His words were spirit and life; and as His disciples our lives must be the sacrament of our message. The natural heart will do any amount of serving, but it takes the heart broken by conviction of sin, and baptized by the Holy Ghost, and crumpled into the purpose of God before the life becomes the sacrament of its message.
There is a difference between giving a testimony and preaching. A preacher is one who has realized the call of God and is determined to use his every power to proclaim God’s truth. God takes us out of our own ideas for our lives and we are "batter’d to shape and use," as the disciples were after Pentecost. Pentecost did not teach the disciples any thing; it made them the incarnation of what they preached – "Ye shall be witnesses unto Me."
Let God have perfect liberty when you speak. Before God’s message can liberate other souls, the liberation must be real in you. Gather your material, and set it alight when you speak.