The term “Service of mankind is service to God” means that selfless service for the cause of humanity is as important as worshiping to God.
Love of mankind is, in Vivekananda’s opinion, the worship of God. This is the gist of all worship is to be pure and to do well to others.
A person who devotes his life for the betterment of the human race is loved by God. If there is any God, He would rather want us to live not only for ourselves but for our fellowmen too.
Great men knew this truth. Life to them was action. This is the service of others. A life lived for selfish gains, regardless of others needs, is worse than death. A bird or a beast collects food for itself and its young ones. It is some kind of work no doubt, but it is not that kind of work that man is expected to do. He is the roof and crown of creation and so he is supposed to live a larger life by applying himself to the service of humanity.
God is omnipresent. He would stand before us everywhere. He would stand by a man in the temple. He will also stand by a man on the plough. As Rabindranath once said in one of his lyrics, God is with the farmer tilling his field, with the laborer laying the path.
We should therefore spurn the age-old practice of searching for Him in the temple of bricks. If God is in me, He is in others too, in the rich and the proud as in the lowly and the lost. To stand on the dust and perspire in the service of others is thus to be in service of God Himself, to worship God in the true sense of the term.