NOTHING IS TO BE ADDED TO THE GOSPEL OF JESUS CHRIST:

The New Testament of our Lord and Savior Jesus Christ; so this second part of the holy Bible is entitled: The new covenant; so it might as well be rendered; the word signifies both. But, when it is (as here) spoken of as Christ's act and deed, it is most properly rendered a testament, for he is the testator, and it becomes of force by his death (Heb. ix. 16, 17); nor is there, as in covenants, a previous treaty between the parties, but what is granted, though an estate upon condition, is owing to the will, the free-will, the good-will, of the Testator. All the grace contained in this book is owing to Jesus Christ as our Lord and Saviour; and, unless we consent to him as our Lord, we cannot expect any benefit by him as our Saviour. This is called a new testament, to distinguish it from that which was given by Moses, and was not antiquated; and to signify that it should be always new, and should never wax old, and grow out of date.

 These books contain, not only a full discovery of that grace which has appeared to all men, bringing salvation, but a legal instrument by which it is conveyed to, and settled upon, all believers. How carefully do we preserve, and with what attention and pleasure do we read, the last will and testament of a friend, who has therein left us a fair estate, and, with it, high expressions of his love to us! How precious then should this testament of our blessed Saviour be to us, which secures to us all his unsearchable riches! It is his testament; for though, as is usual, it was written by others (we have nothing upon record that was of Christ's own writing), yet he dictated it; and the night before he died, in the institution of his supper, he signed, sealed, and published it, in the presence of twelve witnesses. For, though these books were not written for some years after, for the benefit of posterity, in perpetuam rei memoriam--as a perpetual memorial, yet the New Testament of our Lord Jesus was settled, confirmed, and declared, from the time of his death, as a nuncupative will, with which these records exactly agree. The things which St. Luke wrote were things which were most surely believed, and therefore well known, before he wrote them; but, when they were written, the oral tradition was superseded and set aside, and these writings were the repository of that New Testament. This is intimated by the title which is prefixed to many Greek Copies,Tes kaines Diathekes Hapanta--The whole of the New Testament, or all the things of it. In it is declared the whole counsel of God concerning our salvation, Acts xx. 27. As the law of the Lord is perfect, so is the gospel of Christ, and nothing is to be added to it. We have it all, and are to look for no more. (extract from  :  Matthew Henry’s Complete Commentary on the Whole Bible ‘St.Matthew’)

God bless you as you study the Bible.