‘The Spirit itself beareth witness with our spirit, that we are the children of God.’ — Romans 8:16.
But what is the witness of the Spirit?
“The original word μαρτυρία may be rendered either (as it is in several places) the witness, or less ambiguously, the testimony, or the record: So it is rendered in our translation (1 John 5:11), ‘This is the record,’ the testimony, the sum of what God testifies in all the inspired writings, ‘that God hath given unto us eternal life, and this life is in his Son.’ The testimony now under consideration is given by the Spirit of God to and with our spirit: He is the Person testifying. What he testifies to us is, ‘that we are the children of God.’ The immediate result of this testimony is, ‘the fruit of the Spirit;’ namely, ‘love, joy, peace, long-suffering, gentleness, goodness:’ and without these, the testimony itself cannot continue. For it is inevitably destroyed, not only by the commission of any outward sin, or the omission of known duty, but by giving way to any inward sin; in a word, by whatever grieves the Holy Spirit of God.
“I observed many years ago, ‘It is hard to find words in the language of men, to explain the deep things of God. Indeed there are none that will adequately express what the Spirit of God works in his children. But perhaps one might say, (desiring any who are taught of God, to correct, soften, or strengthen the expression,) By the testimony of the Spirit, I mean, an inward impression on the soul whereby the Spirit of God immediately and directly witnesses to my spirit, that I am a child of God; that Jesus Christ hath loved me, and given himself for me; that all my sins are blotted out, and I, even I, am reconciled to God.’
“After twenty years’ further consideration, I see no cause to retract any part of this. Neither do I conceive how any of these expressions may be altered, so as to make them more intelligible. I can only add, that if any of the children of God will point out any other expressions, which are more clear, or more agreeable to the word of God, I will readily lay these aside.
“Meantime let it be observed, I do not mean hereby, that the Spirit of God testifies this by any outward voice; no, nor always by an inward voice, although he may do this sometimes. Neither do I suppose, that he always applies to the heart (though he often may) one or more texts of Scripture. But he so works upon the soul by his immediate influence, and by a strong, though inexplicable operation, that the stormy wind and troubled waves subside, and there is a sweet calm; the heart resting as in the arms of Jesus, and the sinner being clearly satisfied that God is reconciled, that all his ‘iniquities are forgiven, and his sins covered.’”
— John Wesley, Sermon #11 “the Witness of the Spirit: Discourse 2” (II:1-4).