Is the marriage still valid, or is it necessary to remarry after apostatizing from Islam and then returning to it?

We praise Allah for your repentance and ask Him to grant you steadfastness. What we understand from your question is that you apostatized from Islam to marry a non-Muslim woman. In this case, you are not required to renew the marriage contract after embracing Islam, because disbelievers, upon entering Islam, are allowed to maintain their previous marriages, unless there is a lawful impediment that prevents the continuation of the marriage, such as his marriage to one of his mahram (unmarriageable kin). Sheikh Ibn Baz mentioned that the Prophet (peace be upon him) did not command disbelievers who embraced Islam to renew their marriage contracts.