Could anyone shed some light on this?
The easy answer is that we don't believe that we literally become deities, but only use the phrase "become gods" in a rhetorical sense, referring to the paradox of the Incarnation and the Redemption, how God became man so that we men might become more like Him. Therefore, the Protestant assertion is typical -- they assert that we believe something that we do not, casting aspersions and acting suspiciously towards it, in order to justify their unwarranted belief that they have a "purer" and "simpler" religion that is more aligned with their melodramatic anthropomorphisation of God's love and the generosity of Our Lord in the Gospels. Or else they deliberately misinterpret our beliefs or interpret them in an unjustly cynical manner, in order not to refute them, but to give the impression to themselves that they have refuted them.
Divinisation/theosis/sanctification refer to the participation of the man in the state of grace in the life of God, since being justified and having sanctifying grace means that the Holy Trinity is literally dwelling within the soul. Since all good comes from God, any time one commits a good action, one is actually only passively participating in an action of God's grace, and thus Our Lord is actually committing the good action through the soul who responds to Him, who only coöperates in His work. As the soul is involved in more and more of the Divine Life this way, she becomes purified and more attuned to the prompts and desires and vision and presence of the Blessed Trinity Itself. These good actions are not forgotten in Heaven, and are kept in the mind of God as the soul's merit. Therefore, the soul becomes closer to God as she is elevated to Him, and, after death, will (in a certain sense) see all things -- including the self --as God sees them -- this is the Beatific Vision.
Anyway, I doubt most Protestants are open to hearing such things. They will see foul play where they will, since they do not truly believe in the Incarnation, at least not in any meaningful or practical or serious way. They do not believe in spiritual and moral progress. They do not believe in increases and losses of sanctifying grace, or the absolution of sins, or the expiation of sins, or whatever. They think that it is impossible to please God except through having a simplistic and ahistorical "faith" in Him, one which is completely and continually made up by themselves and their vain imaginings. And even within this "faith," God does not actually forgive their sins, but "covers"/ignores them completely, such that none of their sins count. Their false assurance of salvation, therefore, is entirely self-referential and emotional. Theirs is an emotivist religion; thus, since they feel negative things about the True Faith, almost all of their engagements with one defending it will be governed by the dominating ill-feelings they harbour towards their own idea of it. That is to say, it is almost impossible to reason with Protestants, since their whole lives and mindsets and references are already outside the bounds of reason.