Assembly is a noun, and denotes a gathered body. Assembling is a verb, indicating the action of gathering together. To forsake–abandon–the action, or practice, of gathering with fellow-believers is quite different than occasionally missing one such assembly.
When Hebrews 10:21-25 is read in its context you will see the Hebrews were leaving their new found faith in Jesus Christ only to return to the Law of Moses. In today’s churches, I can understand the purpose in warning believers against their infrequent participation in worship services. The less that your around the people of God, you stand a better chance of reverting back to your former ways.
But, to use this passage of scripture to say to the people of God, they will go to hell for missing church services or they must be in attendance every time the church doors are open is misleading and manipulating people.
Preachers and teachers, why not tell and teach the truth and tell the completed store when you tell it and teach it?