Bears witness (4828) (summartureo from sun/syn = with, together, speaks of intimacy + martureo = witness) literally means to bear witness with (or in support of another), to provide credible supporting evidence to one's case by giving testimony or by testifying. To show to be true, give evidence in support of. The net result is to provide support to another. God knows believers need this support in regard to the assurance of their salvation and He makes abundant provision for our need. The present tense emphasizes that the Holy Spirit continually bears witness with our spirit. Summartureo - Used 3 times in the NT - Ro 2:15; 8:16; 9:1 twice referring to the conscience as witness and once in our text referring to the Holy Spirit. Romans 2:15 in that they show the work of the Law written in their hearts, their conscience bearing witness and their thoughts alternately accusing or else defending them, Romans 9:1 I am telling the truth in Christ, I am not lying, my conscience testifies with me in the Holy Spirit, How is the Spirit's witness manifested? Olshausen beautifully says, in His comforting us, His stirring us up to prayer, His reproof of our sins, His drawing us to works of love, to bear testimony before the world, etc. On this direct testimony of the Holy Ghost rests, ultimately, all the regenerate man’s conviction respecting Christ and His work. Barclay writes that Paul... "uses still another picture from Roman adoption. He says that God’s spirit witnesses with our spirit that we really are his children. The adoption ceremony was carried out in the presence of seven witnesses. Now, suppose the adopting father died and there was some dispute about the right of the adopted son to inherit, one or more of the seven witnesses stepped forward and swore that the adoption was genuine. Thus the right of the adopted person was guaranteed and he entered into his inheritance. So, Paul is saying, it is the Holy Spirit himself who is the witness to our adoption into the family of God." Henry Alford asks and explains... “What is this witness of the Spirit itself? All have agreed, and indeed this verse is decisive for it, that it is something separate from and higher than all subjective inferences and conclusions. But on the other hand, it does not consist in mere indefinite feeling, but in a certitude of the Spirit’s presence and work continually within us. It is manifested, as Olshausen beautifully says, ‘in His comforting us, His stirring us up to prayer, His reproof of our sins, His drawing us to works of love, to bear testimony before the world, etc.’ And he adds, with equal truth, ‘On this direct testimony of the Holy Ghost rests, ultimately, all the regenerate man’s conviction respecting Christ and His work.’ ” (The New Testament for English Readers Online) Spurgeon on the testimony of the Spirit... It corroborates the testimony of conscience. We feel that we are the children of God; and the Spirit of God comes forward as a second, but still greater and higher witness, to confirm the testimony that we are the children of God. At this point in Romans 8, Donald Barnhouse issues a solemn warning that all in the Body of Christ, preachers and parishioners, should hear and heed.... WE received the Holy Spirit of God when we were made alive in Christ. One of the first effects of this stupendous and miraculous work is that we know that we have become children of God. We have not received the spirit of bondage that we might relapse into fear, but we have received the Spirit of God in our public manifestation as sons. We immediately are the objects of a spiritual experience. We cry “Abba, Father,” the Holy Spirit Himself testifying along with our human spirit that we are the children of God. Several things about this experience must be examined closely. First, it must be understood that no spiritual experience is valid in itself. Every experience can be counterfeited, and therefore, no experience is valid that is not solidly based in correct theology. It is disastrous to build one’s theology upon some experience; it is always necessary to explain our experience by the Word of God. In the course of the past centuries of church history abundant evidence has been accumulated to show that it is possible to mistake the voice of Satan for the voice of God, and that it is even more possible to mistake the voice of self for the voice of God. That is why we must always be willing to turn to the Word of God, why we must repudiate any voice that speaks contrary to the Word of God, and avoid any experience that is an end in itself. We want no “inner light” experience, “divine voice” experience, we want no ecstatic mysticism that feels rather than thinks. The certain fact of our reception as sons of God must be based objectively on the written Word of God, and then, subjectively, on the fact of the Holy Spirit’s joint witness with our spirit. ... The Holy Spirit does not bear witness to our spirits that we are children of God, but He bears witness with our spirits. The assurance of our salvation does not depend entirely upon the external testimony of the Holy Spirit, but there is an inward conviction which belongs to us, ourselves. We know that we have passed from death unto life; we have the inner conviction that we have become children of God. We find something within ourselves that turns outward and upward to God, so that we find ourselves crying, “Abba, Father.” The Holy Spirit’s testimony joins this testimony of our own inner being, confirms it, so that at the mouth of the two witnesses this thing is established. (God’s Heirs: Romans 8:1—39) THAT WE ARE CHILDREN (born ones) OF GOD: hoti esmen (1PPAI) tekna theou: Spurgeon... What better testimony can we have than that of these two witnesses, first of our own spirit, and then of the Holy Spirit himself, “that we are the children of God”? Note that this is not spoken concerning everybody. The doctrine of the universal Fatherhood of God in a doctrine of the flesh, and not of the Spirit; it is not taught anywhere in God’s Word. This is a Fatherhood which relates only to those who are spiritual; we are born into it by the new birth, and brought into it by an act of grace in adoption. “Beloved, now are we the sons of God;” this is a special privilege that belongs only to those who are spiritual. Many of you make a profession of being the children of God. Can your own spirit say that it is true? And is there, in addition to the witness of the Spirit within you that it is true? If not, unless there is a witness to our testimony, it avails nothing. Our Lord Jesus Christ said, “If I bear witness of myself, my witness is not true”; and if he chooses to put himself on a level, as it were, with the rest of humanity in that respect, we cannot expect that our witness will stand for ought if it stands alone. No, there must be the Spirit himself bearing witness with our spirit that we are the children of God.