it was one line he (More) just couldn't bring himself to cross.
in theory, he had no genuine problems with many elements of Protestantism, indeed, his favourite daughter, margaret, was married to one. he did have a problem with Henry's use of it, however, as he believed, arguably rightly, that Henry was using it to force his own way rather than because of a true theological conflict of the conscience.
and so, the entire problem with his refusing to take that oath lasted years before he was finally executed.
he tried everything a man of his considerable intelligence could think of to get out of it, adhering to the immortal line, 'give unto Caesar what is Caesar's and unto God what is God's.'
thomas and his wife, however, being the clever people they were, did managed to hide most of their considerable assets (dame alice was his second wife, who brought to the marriage her own wealth after her first husband, like thomas's first wife, had died).
thomas was a strong proponent of women's education and the place of women in society, insisting that his three daughters, stepdaughters and his wealthy ward, who later married his only son, were educated to a standard that was remarkable for women of that age.
margaret roper, his daughter, was often termed 'the cleverest woman in christiandom' during her life.
'i die the king's good servant. but God's first.'