Is giving a golden chain with a cross on it to a Christian person considered encouraging them to believe their religion is true? And how can this mistake be rectified?

Presenting a necklace with a cross to a Christian leads to several prohibitions: The man might wear it, and it is forbidden for men to wear, which would be assisting him in a forbidden act. Furthermore, it encourages him in his error, as the cross is a symbol of disbelief. It also shows affection and love towards him, which weakens the doctrine of al-wala' wal-bara' (loyalty and disavowal). What should have been done was to erase the symbol, and one must repent from this action and never return to anything similar.