When God's children give into temptation and fall into sin, their Heavenly
Father assures them that if they will confess their sin to Him (agree with Him that
they have sinned), and turn away from it (repent of it), that He will forgive
them their sin. He didn't say that He might forgive them their sin; He says that He will.1
God knows all things (things past, things present, things future, and things that could be.) He knows when we are sincere, and He knows when we are not. He knows that those who sincerely repent of their sin today, might repeat it tomorrow. Even so, God has said that if one of His children sincerely determines to turn away from their sin, that He will forgive them. We receive parental forgiveness from our Heavenly Father when we sincerely seek it.
1 (1 John 1:9)