This act is considered as one of the greater sins. However, the person doing it - because he has not really become a murtad and is still a believer in Islam - remains a muslim.
Secondly, based on obligatory precaution, permanent marriage with ahle kitab (eg christian) is not permitted. Temporary marriage with ahle kitab, in case of not having a muslim wife, is
not prohibited providing the relevant conditions have been fulfilled.
Tawba and repentance is obligatory and inshaAllah will be forgiven. The reality of tawba is regret from the sinful act carried out. That is why it has been said: Regret is the same as tawba/repentance. Real regret is always accompanied by determination to leave the sin and not to return to it.
Tawba is not achieved by just saying astaghfirullah (I seek forgiveness from Allah). Rather, there should be intense regret and determination to leave the sin forever, and there should be attempt to the fullest possible extent to achieve that.
May Allah increase your tawfiqaat.