What is the ruling on one who swore, made a vow, and mockingly uttered apostasy (without knowing the sinfulness of that statement at the time) to rid himself of masturbation in his youth, then returned to it? Is expiation required for this, and does it affect the validity of his marriage and its contract?

Praise be to Allah, Who has accepted your repentance. Your marriage is valid. Your oath to leave Islam if you did such and such is a forbidden oath (yamin muharram), and according to some scholars, it necessitates expiation (kaffarah), or according to the majority, repentance. The vows you made are vows of contention (nudhur lijaj), and they necessitate either expiation or fulfilling the vow, with the option to choose. What is incumbent upon you – according to the majority of scholars – is an expiation for each oath and vow you broke. You should investigate and expiate in a way that gives you certainty or a strong presumption that you have cleared your conscience. According to the Hanbalis, these expiations overlap, and a single expiation suffices for them. The more cautious approach is that of the majority, though it is permissible to follow the Hanbali jurists as it is easier for you.