‘To me they will cry,
“O God of Israel, we know you.”
Israel has cast off what is good,
The enemy will pursue him.
In its extremity Israel will then call out, ‘O God of Israel we know you'. They would assume that because God was their God, the God of Israel, He must listen to them in their need and respond to their call because they ‘knew His Name'. This would include the idea that they knew how to manipulate Him through the cultus and could thus persuade Him to do what they wanted. And they would make this claim even though Hosea and YHWH had both made clear that that was far from the truth (Hosea 4:1; Hosea 4:6; Hosea 5:4; Hosea 6:6), for had they truly known Him they would have known that they could not manipulate Him and would have obeyed His commandments. Their failure had lain precisely in the fact that they had seen Him as just another nature god, and not as the living God Who required obedience. In other words they had not had a true knowledge of YHWH.
And because they did not know YHWH they had ‘cast off what was good', that is the covenant and the Law and true worship and social justice. Thus the consequence was that ‘the enemy would pursue them'. Pursuit by the enemy was one essential aspect of the curses in Deuteronomy 28:22; Deuteronomy 28:45. Thus the Levitical/Deuteronomic curses are being seen as being fulfilled on faithless Israel.
Some translate as, ‘Israel has cast off the Good One', but there is no precedent for it elsewhere, and ‘casting off what is good' fits the context, and indeed includes the idea of casting off the Good One as part of what is good.
‘O God of Israel.' In the Hebrew text ‘God' and ‘Israel' are divided by the word ‘we know you' (thus producing some of the unusual translations), but in fact the separating up of titles in this way so as to fit in with the metre was a feature of Hebrew poetry.