The Spirit Himself bears witness with our spirit that we are children of God, and if children, then heirs—heirs of God and joint heirs with Christ, if indeed we suffer with Him, that we may also be glorified together.
In a court of law, eyewitness testimony can be very compelling. If corroborated by other evidence, it can carry heavy weight. One reason this is the case in our judicial system is that testimony must be preceded by an oath. The oath lends credibility to the witness because a false witness condemns himself in the eyes of the law if proven to be so. If you were accused of committing a crime you did not commit, you would hope to find a witness who could place you away from the scene of the crime at the time it was committed. Your providing an alibi that you were elsewhere is not nearly as compelling as the corroborating testimony of other people. This is one of the functions of the Holy Spirit for those who believe in Jesus Christ. The Spirit testifies as a witness to their belonging to God.
There is an accuser who roams the Earth today and who tries to condemn us. He does this not only through himself and his demons but also through other people. This is condemnation that comes our way while we are alive and well in this system of things. And on that great day of judgement, we can expect him to bear witness to every sin that we have ever committed. We can expect him to make a case against us entering into the kingdom of God. Although his claims may be factual, his legal argument will not suffice. In fact, the Holy Spirit will bear witness to our inheritance through Jesus Christ. Factually, we have sinned; legally, we have been absolved of that sin. When Satan tries to prove the case against the children of God, the Holy Spirit will testify that we have been redeemed and drawn into the fold of the father. The Holy Spirit will provide the evidence necessary to seal our victory.
There can be no greater witness than the Spirit of God himself. We have the Spirit of an infinite, omnipotent and omniscient being testifying on our behalf that we belong in his kingdom. He not only provides the way for our salvation but also gives the testimony that proves our absolution through him. The Father offered his perfect Son to pay our debt. The Son suffered what we should have suffered to set us free. The Spirit testifies that it is so. This testimony cannot be controverted or proven false. Any argument that attempts to stand against the testimony of the Holy Spirit will fall. That is the case not only on the great day of judgement but also every day that condemnation tries to take a foothold in us here on Earth. Father, remind us of the fact that your Spirit testifies to our belonging to You, and help us to renew our belief in that testimony daily.