'Witness' in the Bible

For God is my witness, whose servant I am in spirit in the good news of his Son, that you are at all times in my memory and in my prayers,

Because the work of the law is seen in their hearts, their sense of right and wrong giving witness to it, while their minds are at one time judging them and at another giving them approval;

But now without the law there is a revelation of the righteousness of God, to which witness is given by the law and the prophets;

And he was given the sign of circumcision as a witness of the faith which he had before he underwent circumcision: so that he might be the father of all those who have faith, though they have not circumcision, and so that righteousness might be put to their account;

The Spirit is witness with our spirit that we are children of God:

I say what is true in Christ, and not what is false, my mind giving witness with me in the Holy Spirit,

For I give witness of them that they have a strong desire for God, but not with knowledge.