Who are those that are called??
AMP - 28 We are assured and know that [God being a partner in their labor] all things work together and are [fitting into a plan] for good to and for those who love God and are called according to [His] design and purpose.
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I don't really understand. Does it mean that there are people that are not called according to his purpose? And if yes, why?
I would say that, as Paul elaborates in the next verse (Romans 8:29), God's purpose did not pertain to the call itself (which is extended to all), but consisted of conforming those individuals who would respond to the call (as God knew they would from eternity past) to the likeness of Christ.