Constantine is perhaps best known for being the first Christian Roman Emperor. His reign was a turning point for the Christian Church. In 313 Constantine announced toleration of Christianity in the Edict of Milan, which removed penalties for professing Christianity (under which many had been martyred in previous persecutions of Christians) and returned confiscated Church property. Though a similar edict had been issued in 311 by Galerius, then senior emperor of the Tetrarchy, Galerius' edict granted Christians the right to practice their religion but did not restore any property to them.
Scholars debate whether Constantine adopted his mother St. Helena's Christianity in his youth, or whether he adopted it gradually over the course of his life. Constantine was over 40 when he finally declared himself a Christian. Constantine however still maintained the title of Pontifex Maximus, which emperors bore as heads of the pagan priesthood. Writing to Christians, Constantine made clear that he believed he owed his successes to the protection of the Christian High God alone.Throughout his rule, Constantine supported the Church financially, built various basilicas, granted privileges (e.g. exemption from certain taxes) to clergy, promoted Christians to high ranking offices, and returned property confiscated during the Great Persecution of Diocletian.His most famous building projects include the Church of the Holy Sepulchre and Old Saint Peter's Basilica.
So yes he was a Christian