'It is I' in the Bible

Did you suffer so many things in vain, if it is indeed in vain?

Because it is in the Writings, that Abraham had two sons, one by the servant-woman, and one by the free woman.

For it is in the Writings, You who have never given birth, be glad; give cries of joy, you who have had no birth-pains; for the children of her who has been given up by her husband are more than those of the woman who has a husband.

Notice, it is I, Paul, who tells you that if you receive circumcision [as a supposed requirement of salvation], Christ will be of no benefit to you [for you will lack the faith in Christ that is necessary for salvation].