To the worker for God the record of these angel visits should bring strength and courage. Today, as verily as in the days of the apostles, heavenly messengers are passing through the length and breadth of the land, seeking to comfort the sorrowing, to protect the impenitent, to win the hearts of men to Christ. We cannot see them personally; nevertheless they are with us, guiding, directing, protecting. {AA 152.4}

Mat 5:43 Ye have heard that it hath been said, Thou shalt love thy neighbour, and hate thine enemy.
Mat 5:44 But I say unto you, Love your enemies, bless them that curse you, do good to them that hate you, and pray for them which despitefully use you, and persecute you;
Mat 5:45 That ye may be the children of your Father which is in heaven: for he maketh his sun to rise on the evil and on the good, and sendeth rain on the just and on the unjust.

The "guardian angel" statement may be meant more as something to keep in mind than an implication that unbelievers are without protection.

That's a good thought.