In human reasoning, demonstration cannot usually be obtained except in mathematics, but not so in divine. There grace outshines and exceeds nature, for the teaching and testimony of the blessed Spirit is always demonstrative, that is, convincing beyond the possibility of doubt. It is not demonstration simply we require, not demonstration of the word, as if there were some innate proof and power in the word itself to demonstrate its own truth, though doubtless it is so when the Spirit shines upon it, but it is the "demonstration of the Spirit." This is very necessary to observe, for you will often hear the word of God spoken of, as if the Bible possessed not only demonstrative proof of its own inspiration, but was able to give that demonstration to the souls of men.
But the demonstration not of the word but of the Spirit in, through, and by the word, is the thing wanted to convert sinners and satisfy saints. This is proof indeed, not cold and hard like mathematical demonstration, but warm, living, softening, and sanctifying, being the very light, life, and power of God himself in the soul. Now Paul's preaching was this demonstration of the Spirit. The Spirit of God speaking in him and by him, so demonstrated the truth of what he preached that it came, as he elsewhere speaks, "not in word only, but also in power, and in the Holy Ghost, and in much assurance" (1 Thess. 1:5).
There are now no Pauls; and, yet, unless we have a measure of the same demonstration of the Spirit, all that is said by us in the pulpit drops to the ground; it has no real effect; there is no true or abiding fruit—no fruit unto eternal life. If there be in it some enticing words of man's wisdom, it may please the mind of those who are gratified by such arts; it may stimulate and occupy the attention for the time; but there it ceases, and all that has been heard fades away like a dream of the night; and, as regards the family of God, we may apply to all such preaching the words of the prophet: "It shall even be as when an hungry man dreameth, and, behold, he eateth; but he awaketh, and his soul is empty: or as when a thirsty man dreameth, and, behold, he drinketh; but he awaketh, and, behold, he is faint, and his soul hath appetite" (Isaiah 29:8).
But anything which is communicated by the Holy Ghost, which is demonstrated by the Spirit to your soul, which is brought into your heart with light, life, and power, sealed and witnessed by that sacred Teacher and divine Comforter; that abides, you take it home with you; it comforts you, not only at the time, but when you look back to it in days to come; it is a bright spot in your soul's experience, when you can believe that then and there God was pleased to bless his word to your soul, and seal it home with a sweet influence upon your conscience. This is "demonstration of the Spirit."
And where there is this, there is "power:" for the Apostle adds, "and of power." The grand distinguishing mark of the kingdom of God is that "it is not in word, but in power." Thus power is given to believe in the Son of God, and we cannot believe truly and savingly in him till power is put forth; power to receive the Lord in all his covenant characters and gracious relationships in the gospel of his grace; power to believe that what God has done he does for ever; power to come out of every doubt and fear into the blessed light and liberty of the truth which maketh free.