Here are some powerful word from some dead theologians who wrote some fantastic books! Roland Allen was ahead of his time in his missiology and decidedly perceptive in addressing the mandate to evangelize.
“What carries conviction is the manifest disinterestedness of the speaker. He speaks from his heart because he is too eager to be able to refrain from speaking. His subject has gripped him. He speaks of what he knows, and knows by experience. The truth which he imparts is his own truth. He knows its force. He is speaking almost as much to relieve his own mind as to convert his hearer, and yet he is as eager to convert his hearer as to relieve his own mind; for his mind can only be relieved by sharing his new truth, and his truth is not shared until another has received it. This his hearer realizes…. To all this is added the mysterious power of a secret. Christian experience is always a secret; and the man who speaks of it to another always pays him a subtle compliment when he entrusts him with his secret of life… In speaking of it he goes through it again; in setting it before another he sets it before himself in a new light. He gets a deeper sense of its reality and power and meaning. In speaking of it he pledges himself to the conduct and life which it involves. He proclaims himself bound by it, and every time that his speech produces an effect upon another, that effect reacts upon himself, making his hold upon his truth surer and stronger.” The Spontaneous Expansion of the Church, Roland Allen, pp. 10,11
“All-important is a diligent endeavor to have the power of the truths professed and contended for abiding upon our hearts, that we may not contend for notions, but that we have a practical acquaintance within our own souls. When the heart is cast indeed into the mould of the doctrine that the mind embraceth — when the evidence and necessity of the truth abides in us — when not the sense of the words only is in our heads, but the sense of the thing abides in our hearts — when we have communion with God in the doctrine we contend for — then shall we be garrisoned by the grace of God against all the assaults of men…. What am I the better if I can dispute that Christ is God but have no sense or sweetness in my heart from hence that he is a God in covenant with my soul?” The Works of John Owen, Vol 12, p. 52, John Owen