。
It is our duty to love our neighbors.
Literal
Neighbor [object-を] love [nominalizer-のは] our duty [copula-だ].
A biblical-adjacent maxim translated into Japanese. 隣人 ('neighbor, fellow human'). ~のは~だ is the cleft sentence ('what does X is Y' / 'it's Y that does X'), foregrounding 隣人を愛するの as the focused element. つとめ ('duty, obligation') can be written 務め or 勤め depending on the sense — here 務め (obligation, one's duty).