Christ is clear as to who is with Him and who is not. There is no middle ground for following Christ. These words indicate that one must choose to be with Christ in order not to be against Him. If one does not choose to follow Christ, one is against Him.
Why? Because unless Christ is allowed to be Lord of our lives, other things and people become lord. It has struck me lately how many people I meet daily are not in sanctifying grace. I noted here on the blog that we cannot trust those who are not in sanctifying grace. Being in deception means that one can only think and follow lies. One has lost discernment and the four cardinal virtues. Or, at the very least, these are blocked.
Those who live under the thrall of other gods will only lead us astray if we are not strong in Christ. And, the second statement has to do with not gathering. To gather means several things, like bringing people into the Church, it means evangelizing openly and praying for converts.
It means following the teaching of Christ.
If we are not gathering we are scattering. Think how.