Summary: For God to be unfaithful it would mean that he would be acting contrary to His nature, which would be impossible: Numbers 23:19 is plain enough: "God is not a man, that He should lie; neither the son of man, that He should repent."
Deuteronomy 7:9 Text
"God, the faithful God"
When we read this quotation we must understand that it is taken from 2 passages of scripture that focus on describing to us one of the 8 characteristics of God which is Faithfulness or Loyalty in Exodus 20 we read: (v.6)
1 God spoke all these words: 2 I am God, your God, who brought you out of the land of Egypt, out of a life of slavery. 3 No other gods, only me. 4 No carved gods of any size, shape, or form of anything whatever, whether of things that fly or walk or swim. 5 Don’t bow down to them and don’t serve them because I am God, your God, and I’m a most jealous God, punishing the children for any sins their parents pass on to them to the third, and yes, even to the fourth generation of those who hate me. 6 But I’m unswervingly loyal to the thousands who love me and keep my commandments.
In Exodus 34 Moses Makes New Tablets (v.6 and 7)
1 God spoke to Moses: "Cut out two tablets of stone just like the originals and engrave on them the words that were on the original tablets you smashed. 2 Be ready in the morning to climb Mount Sinai and get set to meet me on top of the mountain. 3 Not a soul is to go with you; the whole mountain must be clear of people, even animals - not even sheep or oxen can be grazing in front of the mountain." 4 So Moses cut two tablets of stone just like the originals. He got up early in the morning and climbed Mount Sinai as God had commanded him, carrying the two tablets of stone.
5 God descended in the cloud and took up his position there beside him and called out the name, God. 6 God passed in front of him and called out, "God, God, a God of mercy and grace, endlessly patient - so much love, so deeply true - 7 loyal in love for a thousand generations, forgiving iniquity, rebellion, and sin. Still, he doesn’t ignore sin. He holds sons and grandsons responsible for a father’s sins to the third and even fourth generation." 8 At once, Moses fell to the ground and worshiped, 9 saying, "Please, O Master, if you see anything good in me, please Master, travel with us, hard-headed as these people are. Forgive our iniquity and sin. Own us, possess us."
The Covenant Renewed (v.10)
10 And God said, "As of right now, I’m making a covenant with you: In full sight of your people I will work wonders that have never been created in all the Earth, in any nation. Then all the people with whom you’re living will see how tremendous God’s work is, the work I’ll do for you.
We must understand that God the faithful God he alone is sovereign Lord,and He is reliable, trustworthy; what he said, he will do because He say’s He is the "keeper of the covenant and lovingkindness",he is faithful to His promise to be Lord of Israel, and to that promise he says he will be utterly loyal. God has a Steadfast love for His people Israel this is why He entered into a covenant of love with His people in Psalms 25:10 God says
"All the paths of the LORD are mercy and truth unto such as keep his covenant and his testimonies."In Psalms 85:10 God says, "Mercy and truth are met together; righteousness and peace have kissed each other."
When God talks about His steadfast love He is talking about the fact that He has a deep,lasting affection toward His people that is based on relationship.God only has a relationship with those in whom He is in covenenant with and he is faithful to those who are under His covenant.This is what the Hebrew word "Hesedh," speaks of it is a word that has a legal background in that it refers to the obligations involved in a covenant relationship.God’s loyalty to his promises in the covenant is His "hesedh",but since the nation of Israel was a breaker of the covenant there was no obligation whatever on God’s pat to continue his loyalty.Yet the remarkable thing was that He did so.So when "hesedh" is used of God, the word becomes one of the primary expressions in the O.T. to describe the undeserved divine mercy and grace of God. Yet God also shows us another aspect that if Israel "hate him" i.e. be disloyal to him as to disregard him or openly defy him God lets them know that "I am a Jealous God". To God He was betrothed to Israel i.e. Israel was his bride. In Hosea 2:20 in the Message Bible God says "Yes, I’ll marry you and neither leave you nor let you go. You’ll know me, God, for who I really am. " God wanted them to respond in faithfulness to him because he had acted faithfully to them through covenant. This is why he says in Proverbs 3:3 " Let not mercy and truth forsake thee: bind them about thy neck; write them upon the table of thine heart." Those who believe in God need to exhibit faithfulness and steadfastness in their lives. Because God is Faithful!