It is important for Christians to stand up for religious freedom for people of all faiths, not just Christianity, because Christianity is true. Because Christianity is true, its validity will prove self-evident in a free marketplace of ideas.
The suppression of religious freedom or any kind of “wrongthink” is a twofold toxin. First, the suppression of ideas implicitly admits (whether truly or not) that the established orthodoxy cannot stand under scrutiny, and can only survive via handicapping all other competitors. Second, suppressed ideas tend to receive sympathetic attention, turning heretics into martyrs. As Barbara Streisand could attest, the suppression of information and ideas only boosts interest in it.
Martin Luther often quipped how to laugh at the devil was the surest way to scorn him. If bad ideas and false religions are allowed to compete on their own merits, their absurdity becomes publicly evident and shall be quickly undone.