こととて

こととて means "since", "as it was": it gives a reason, nearly always as an excuse for a shortcoming or as grounds for asking to be forgiven. It follows a verb or an い-adjective in the plain form, a な-adjective with な, and a noun with の. It is literary, and you meet it in formal letters, apologies and speeches, not in conversation.

The second half is usually an apology or a plea: おゆるし許しください, ごようしゃ容赦ください, おわびお詫びもうしあげます申し上げます. A negative often takes the old form ぬ: しら知らぬこととて.

こととて or ので

ので and から are the everyday ways to give a reason, and they work for any reason at all. こととて only fits a reason that excuses something: inexperience, not knowing, short notice.

Without こと, とて after a noun means "even": こども子供とて, "even a child". That is a different pattern.