That depends on how involved humanity is in cultivating that atmosphere. There have been proposals to, for example, redirect comets/near-Mars objects that are i

That depends on how involved humanity is in cultivating that atmosphere. There have been proposals to, for example, redirect comets/near-Mars objects that are icy/have oxygen or other desirable materials to burn up in the Martian atmosphere, adding to it. There are likely other ways we can affect the atmosphere. From a terraforming standpoint, a viable, long-term magnetosphere is necessary in the medium and long term. In the short term, humans would be stuck in shielded suits and indoors, both to protect from radiation and to ensure a breathable atmosphere.