For the most part, Christians did not influence the spread of Islam in any direct way. There are some ways and some regions in which Christians slowed or stopped the spread of Islam. There are also indirect ways in which Christians may be said to have helped Islam spread.
Christianity can be said to have halted the spread of Islam militarily and socially. Once Muslim armies reached the edges of Europe (as in the Balkans) they had a hard time making any headway. These were areas where Christian faith (and armies) were strong and Islam was unable to spread through these areas.
However, in the places that Islam did reach, you can argue that Christians helped it spread. You can argue that Christianity (and Judaism) had helped get people used to the idea of monotheism. This might have made Islam seem less of a strange and novel faith and allowed it to spread more easily in the areas where it did take hold.