Being faithful is obeying his commandments.
The sad honest truth is that it is our inclination not to obey his commands.
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Deu 31:19 "Now therefore write this song and teach it to the people of Israel. Put it in their mouths, that this song may be a witness for me against the people of Israel.
Deu 31:20 For when I have brought them into the land flowing with milk and honey, which I swore to give to their fathers, and they have eaten and are full and grown fat, they will turn to other gods and serve them, and despise me and break my covenant.
Deu 31:21 And when many evils and troubles have come upon them, this song shall confront them as a witness (for it will live unforgotten in the mouths of their offspring). For I know what they are inclined to do even today, before I have brought them into the land that I swore to give."
The real question is why aren't we always obedient if it's not hard? We are told by God Himself that our inclination is to be disobedient. The commandments serve as a testimony to our disobedience. It exposes our true nature. This is our sin nature and our need for our Lord Jesus. We pled with the Spirit to help conform us to the obedience of His word and to quench our rebellious nature.