Is it a sin? Yes it is. Life in Christ was meant to be lived in community and not in isolation. Sometimes isolation is a product of unavoidable circumstance; in that case it is not a sin. However, to deliberately isolate yourself from God's people is a sin.
Mind you, there is still a requirement to ensure that the church you choose teaches what the Bible really teaches and not the myths of popular Christianity. It is better to remain apart than to support a community which teaches a false gospel. Having said that, once you have found a fellowship which believes God's word and practices it, you would be commiting a sin to stand aloof from it.
Here is how the writer to the Hebrews puts it:
Hebrews 10:23-25 Let us hold fast the confession of our hope without wavering, for He who promised is faithful. 24 And let us consider one another in order to stir up love and good works, 25 not forsaking the assembling of ourselves together, as is the manner of some, but exhorting one another, and so much the more as you see the Day approaching.
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