Gentiles were now being grafted into the covenanted olive tree of Israel (Rom. 11:17). These Gentiles were grafted in alongside believing Jews who had been there, in that same tree, since infancy. While leaving the substance of the covenant itself untouched (we must remember it was an everlasting covenant), our sovereign Lord determined that He would alter the external sign of the Abrahamic covenant. Before, that sign had been circumcision. Christ now declared that the means of sealing the nations promised to Abraham (Rom. 4:13) was to be accomplished through baptism (Matt. 28:19).
