Complex sentence: While a Christian, Tertullian denounced Christian doctrines he considered heretical, but it has been claimed that later in life converted to Montanism, a heretical sect that appealed to his rigorism.
Split into simpler sentences: While a Christian, Tertullian denounced Christian doctrines he considered heretical, but later in life adopted views that themselves came to be regarded as heretical. Later in life, Tertullian is thought by most to have joined the Montanists, a heretical sect that appealed to his rigorism.