Sirhan Sirhan was Christian and he attended a Lutheran school.
In Islam, Taqiya or Taqiyya is a precautionary dissimulation or denial of religious belief and practice in the face of persecution.
Sirhan Sirhan was definitely anti-Catholic. He never abandoned the early teachings of his childhood, the twig was already bent before he ever got as far as any formal schooling. No matter what the explanations and rationalizations later on, he was never really a Christian.