"He who is not for Me is against Me, and he who does not gather with Me scatters abroad." -Matthew 12:30
Jesus makes a very strong statement here! Let's look at both parts of it.
"He who is not for Me is against Me..."
Jesus made it abundantly clear that there is no middle ground when it comes Him. You're either for Him or against Him! Unfortunately, lots of people think they can remain neutral on the subject. If you ask them about Jesus, they'll tell you that they think He was a pretty cool guy -- but that they're not really into living a gung-ho Christian lifestyle. They give lip service to Him, but they are not actively "for Him." According to Jesus, that means they are actually against Him. This is an serious warning to anyone who is still "on the fence" about Jesus!
"...and he who does not gather with Me scatters abroad."
This is a serious warning to the believer! Lots of Christians want to follow Jesus, but don't actively participate in work of drawing others into fellowship with Him. According to Jesus, that means they might as well be driving people away from God! All believers are charged with the "ministry of reconciliation," and we are all responsible for gathering the great harvest of souls.