Every person alive when Christ returns, not to mention every person who is in heaven or in hell will confess that Jesus Christ is Lord (to the glory of the Father) and bow down to Him.
Imagine those who denied Him, blasphemed Him, ridiculed Him, crucified Him, hated Him bowing down to Him and confessing that He is Lord. Atheists, Roman Catholics, Muslims, Hindus, New Agers, the entire plethora of unbelievers will too finally confess He is Lord and bow down to Him – but it will be too late for them.
So it’s safe to say that Muslims, Atheists, Roman Catholics, Muslims, unsaved denominationalists, New Agers, et al will one day too be Christians – they just don’t know it yet 😉
Imagine spending your lifetime fighting against God and Biblical truth, only having to acquiesce one day when it’s too late.