I think so. Explorers like Leif Erickson weren't hostile towards people of other races, nor did they try to conquer their lands or commit genocide. Before Christianity, most Europeans (bar Romans & Greeks) kept to themselves for the most part & weren't cruel to people of other races unless provoked.
"Colonialism: the policy or practice of acquiring full or partial political control over another country, occupying it with settlers, and exploiting it economically."
Europe has always had colonialism. The Greeks colonized many areas of the Mediterranean, including southern Italy, the Romans' expansion established settlements across 'barbaric' Europe, the migration of germanic tribes could be argued as being colonialism. Vikings colonized regions of Britannia, killing the local Anglo-Saxons (who themselves are derived from Germanic tribes invading the island) as well as the Celts. All these examples, aside the Germanic migration and Viking Conquest (which weren't connected to the religion), predates Christianity.
The Colonial Era was caused by millennia of colonial culture suddenly being accelerated by easily claimable land, could religion have played a role in a superiority complex? Possible, most things are connected, but I wouldn't say it was the main reason it happened.