[Therefore ye need not suppose that ye can turn the right hand of the Lord unto the left]: This is an idiomatic expression from the Old Testament that references changing. In this context, we are to understand that the Lord does not change. In the following passage the phrase indicates that we should not deviate from following the law and statutes of God:
Deuteronomy 17:19-20
19 And it shall be with him, and he shall read therein all the days of his life: that he may learn to fear the LORD his God, to keep all the words of this law and these statutes, to do them:
20 That his heart be not lifted up above his brethren, and that he turn not aside from the commandment, to the right hand, or to the left: to the end that he may prolong his days in his kingdom, he, and his children, in the midst of Israel.
[that he may not execute judgment unto the fulfilling of the covenant]: This is what the Lord will not fail to do. He will be undeviating in the fulfillment of the covenant with Israel.
Textual: This is the end of a chapter in the 1830 edition.