i think the question should be whether God's people can be guilty of sin, not whether God has completely forsaken His chosen people -- because God has said He will never completely forsake her, though He now grafts us unworthy gentiles into His tree.
have a look here:
Remember me, O Lord, when you show favor to your people;
help me when you save them,
that I may look upon the prosperity of your chosen ones,
that I may rejoice in the gladness of your nation,
that I may glory with your inheritance.
Both we and our fathers have sinned;
we have committed iniquity; we have done wickedness.
(Psalm 106:4-6)
and further down in the Psalm...
They sacrificed their sons
and their daughters to the demons;
they poured out innocent blood,
the blood of their sons and daughters,
whom they sacrificed to the idols of Canaan,
and the land was polluted with blood.
Thus they became unclean by their acts,
and played the whore in their deeds.
Then the anger of the Lord was kindled against his people
(Psalm 106:37-40)
so here the psalmist describes the sin of Israel, and yet calls them God's people.
this psalmist too -- did David not shed innocent blood when he had Uriah the Hittite killed?
any time you see someone who professes Christ who hates someone in their heart, Jesus said this is as much a sin as murder. do we say 'that person is no longer a Christian'? or do we say that they have sin in their heart, and ought to repent? they aren't walking in Christ, but does that mean the one who said I will never leave you nor forsake you has forsaken them? a lost sheep remains a sheep.
some will say that a true Christian is completely holy and blameless and never sins. i suppose that means there are none -- because John said if we say we are without sin, we lie. it is Christ's imputation of righteousness that makes us holy, and His forgiveness and the baptism into His death and resurrection that wipes away all our sin -- not our own righteousness. in an existential sense, we are pure and spotless, and with Christ as our advocate, so are we judged. but in a tangible sense, we still daily need to take up our cross, and to seek the mercy of God.
all facts of the war in Gaza aside, God has chosen Israel to bear His name, and Jerusalem as His city. He has brought us also into His household, but He hasn't made Rome or London or New York the city that bears His name. Israel today is still blinded and in rejection, and God may chastise them as He sees fit. He's said that in the last days every nation will be against Jerusalem, and Jerusalem will be a stumbling stone to all of them - and Christ Himself, when He returns, will defend her against the whole world.
even were i to agree that Israel does wrong by what they are doing today (and i don't - but this is about God, not the actions of the Jews) i will never curse Israel. if she sins, i will mourn for her, but she is the beloved of God, and i dare not be against her as a nation.