Was Christianity spread by force?

Not primarily, and not only. The honest answer sits between two oversimplified stories, and depends heavily on which period and region you're asking about.

For its first three centuries, Christianity had no political or military power at all. It spread through preaching, personal relationships, and communal appeal—often while being actively persecuted by Rome, not while enforcing anything on anyone.

That changed once Christianity gained state backing, starting in the fourth century. In some later periods and places, coercion genuinely was involved: forced conversions in specific medieval contexts, and colonial-era missionary efforts sometimes backed by political and military power.

So neither extreme holds up. "Always spread peacefully" ignores real historical coercion in some eras. "Always spread by the sword" ignores three centuries of growth with no power to compel anyone, plus enormous peaceful growth in many other times and places. Different periods genuinely had different histories.