Christ's argument in John 6:43-45 would fail if many who are drawn do not believe
Surely, if many of those whom the Father's grace draws and God teaches inwardly still do not believe in Christ or come to him, how could Christ give, in that same passage, this as the reason why not everyone who hears the gospel comes to him, namely that those who come to him must be drawn and taught by the Father?
'Do not grumble among yourselves,' he says. 'No one can come to me unless the Father who sent me draws him. It is written in the Prophets: And they will all be taught by God. Everyone, then, who has heard and learned from the Father comes to me.'
If the very people who do not believe the gospel are drawn by the Father's grace, it would make no sense to say: 'Do not be surprised that this person and that, to whom the gospel was preached, have still not believed it, since those who believe the gospel must be drawn by God's grace.'
Latin
XLI. Et certe si multi eorum qui per Patris gratiam trahuntur & intus a Deo docentur, in Christum tamen non credunt, nec ad eum veniunt, quomodo Christus ibidem hanc rationem afferret cur non omnes qui audiunt Evangelium ad ipsum veniant; quod, scilicet, necesse sit illos qui ad ipsum veniunt a Patre trahi & doceri? Nolite, inquit, murmurare inter vos. Nemo potest venire ad me, nisi Pater qui misit me traxerit eum. Est scriptum in Prophetis, Et erunt omnes docti a Deo. Omnis ergo qui audivit a Patre & didicit venit ad me. Profecto si illi ipsi qui non credunt Evangelio per Patris gratiam trahuntur, absurdum esset dicere, Ne miremini quod isti & illi quibus praedicatum fuit Evangelium in Evangelium tamen non crediderunt; quoniam necesse est eos qui credunt in Evangelium per gratiam divinam trahi.