Complex sentence: Tekuder was born Nicholas Tekuder Khan and had been baptized in his childhood as a Nestorian Christian; however, Tekuder later converted to Islam Steven Runciman.
Split into simpler sentences: Tekuder was born Nikola ("Nicholas") Tekuder Khan, and was baptized in his childhood as a Nestorian Christian. However, Tekuder later converted to Islam Steven Runciman.