I Corinthians 1:9 "God is faithful, by whom ye were called unto the fellowship of his Son Jesus Christ our Lord."
We know that God is faithful. He cannot help but be faithful.
When we are tempted to sin He is able. I Corinthians 10:13 "There hath no temptation taken you but such as is common to man: but God is faithful, who will not suffer you to be tempted above that ye are able; but will with the temptation also make a way to escape, that ye may be able to bear it." Do we always choose to do the right thing when we are tempted? No we don't. If you say you do then you're a much better Christian than I am. I know that I desire to be faithful to make the right choices. God is faithful though not to allow us to be tempted above what we are able to resist. If we commit that sin it is not His fault. We are the ones who choose to give in to that temptation.
But when we do sin....He has promised to be faithful. I John 1:9 "If we confess our sins, he is faithful and just to forgive us our sins, and to cleanse us from all unrighteousness." I'm afraid we're not always faithful to confess our sins. I've always heard it to be best to keep a short sin list with God. In other words...when we sin we should confess it immediately. Don't allow the sin we commit to build up.
Do you realize that even when we're unfaithful to Him He always remains faithful to us. He does not say "You were not faithful to me today so I'm going to choose not to be faithful to you." I Corinthians 1:8 "Who shall also confirm you unto the end, that ye may be blameless in the day of our Lord Jesus Christ." He promises to continue to work in our lives until we go home to be with Him.
He has been faithful to me from the day I got saved till now. I have no reason to doubt He won't continue to be faithful till I go to be with Him.