If one is a Christian how does he not "legislate without injecting his religion". ?
All US citizens, officials & legislators are bound by oath to support the US Constitution, our supreme law. -- One of which says legislators shall make no laws respecting religious establishments.
Can a Christian serve two masters?
If he can't swear an oath to support & defend the Constitution, I'd guess not.
You expect that a Christian would be able to ignore the morals taught by Christ that he should obey and use another standard by which to measure. What standard might that be ? Would a Christian still be a Christian if he did not measure his descions by Christ's teaching?