Deuteronomy 7:9

Know therefore that the Lord your God is a Judge, strong and faithful, keeping covenant and mercy with them who love Him and keep His commandments unto a thousand generations,

He is the Almighty He is truly the Almighty, because He is both powerful and faithful. This point is made in reference to “the oath” [: 8]; for one can be faithful, but powerless to fulfill one’s oath, just as one can be powerful, yet faithless.

He keeps the covenant, and the kindness, for His beloved No one has higher stature than they

and for them who keep His commandments they are second in stature or He keeps the covenant, and the kindness, for them who love Him in their hearts …them who keep His commandments in their speech and in their actions ...

a thousand generations “Thousand” here denotes “infinite”. Some people have incorrectly seen here an allusion to the theory that the planetary cycles repeat themselves every thirty-six thousand years. In fact, they repeat themselves approximately every seventy years.

KNOW THEREFORE from the [circumstances of the] departure from Egypt THAT THE ETERNAL THY G-D, HE IS G-D Who created heaven and earth, as I have explained. And know furthermore from this that He is THE FAITHFUL G-D Whose word shall not return void, and WHO KEEPETH THE COVENANT AND MERCY WITH THEM THAT LOVE HIM, AND KEEP HIS COMMANDMENTS, who fear His Name, TO A THOUSAND GENERATIONS of their seed, just as He kept for you the covenant of your fathers. And know further that He repayeth them that hate Him the recompense of their wickedness to the face of each one of them to destroy them, just as He destroyed the Egyptians and did not delay [in bringing punishment] to them. Now this attribute that he mentioned is true forever. Thus although there is a wicked man that prolongeth his life in his evil-doing, it happens to him only because of this above-mentioned attribute that He is the Keeper of Mercy, and he [the wicked] has done some good which must be recompensed. If so, there are among the attributes of the Holy One, blessed be He, but two ways [of dealing with people]: He repays good for good, and evil for evil.
It is possible that those that hate Him [mentioned here as destined to be destroyed without delay] are those confirmed sinners who deny His existence and have no merit at all. As the Rabbis have said: “Rabbi Yashiyah said: Because of three things the Holy One, blessed be He, is long-suffering to the wicked in this world: they may repent, or they have observed commandments for which the Holy One, blessed be He, repays them their recompense in this world, or perhaps righteous people will descend from them. For so we find that He was long-suffering with Ahaz, and Hezekiah came from him; so also to Amon,, 21:19-22. and Josiah came from him; and so also to Shimei, and Mordecai came from him.” Thus all these [three above categories] are not included in [the term] those that hate Him. Now there is a secret in the expression ‘el panav’ (to his face) [And He repayeth ‘them’ that hate Him ‘el panav’ — “to his face”]], why does it change to the singular [to ‘his’ face, to destroy ‘him’]? Ramban alludes to the secret of the transmigration of the soul, so that at some future generation the recompense is still to “the face” of the sinner after he has been rewarded for the good he has done (Abusaula). as applying to a wicked man who has a good [i.e., a happy and prosperous] life. And then he returns to say, Thou shalt therefore keep the commandment etc., meaning: “therefore keep the commandment, and the statutes, and the ordinances so that you should not be among those who hate Him, who are destroyed, but instead you should be of those that love Him and keep His commandments who are remembered for good and for life.”
Eikev

וידעת כי ה' אלוקיך הוא האלוקים, in the event that you might says: “seeing that G’d has to keep His oath to our patriarchs, why do we have to bother to keep His commandments, seeing that he has to give us this land under any circumstances? Moses answers such a theoretical question before it is even mouthed by telling the people that G’d does not have to let them have the land as an heritage, Although, He has sworn to give the land to Avraham’s and Yaakov’s descendants, He would not violate His oath if this generation of Jews were not to take possession of this land forthwith. It is one of G’d’s attributes that He sometimes lets a long time elapse before carrying out anything He has said which was not tied to a date. Just as He remembers loving kindness performed by His creatures for 1000 generations, so, not lapsing any reward due to the descendants of a deserving ancestor, so He may decide that you are the generation which will be the beneficiary of the promise and oath mentioned. Moses promises that the people whom he addresses will indeed become these beneficiaries with the proviso that they keep G’d’s commandments.

הא-ל הנאמן, Who is constant, not subject to change.

שומר הברית ומשלם לשונאיו, the meaning of the words רשע וטוב לו, [a favourite argument of the heretics who point to the afflictions suffered by the pious and the good life enjoyed by the pious in this life. Ed.] can be understood on two levels: 1) the “good” the wicked appear to enjoy in this life is due to merits accumulated by their forefathers who had not been recompensed for these as yet by G’d. A prominent example of this is the enjoyable life lived by Ishmael who benefited by Avraham’s merits. G’d Himself referred to this in Genesis 21,13 when He said כי זרעך הוא, “because he is your seed. 2) The wicked person himself has accumulated some merits; seeing that on balance he does not qualify for life in the hereafter, G’d has to compensate him while he is on earth. This He does by letting him enjoy the transient joys available in this life to the extent his merits entitle him to.