Philippians 1:1 says, "Paul and Timothy, servants of Jesus Christ"
Since Jesus is not here but seated above in heaven, how do his people get a sense or assurance of really serving him?
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In Jesus' own words, we serve Him by serving our fellow man as He commanded. This is, perhaps, clearest in the following passage:
We also serve Him by keeping His commandments, and by fulfilling the Great Commission.
His physical, visible presence isn't required for us to serve Him. That's true in many cases outside the realm of Christianity. Do we not serve our soldiers by ensuring their families' needs are met while they are overseas? Do they not serve us, and our country though they can't see us? I could list many other examples, but you can see the point.
I have posted on other questions that where close to this, but I'll post here also :).
Even though we can't see god, we can and do serve him. Jesus when he was alive set the perfect example as both a teacher and preacher of gods word. He talked to crowds both small and large to tell them about gods will for the earth and those on it. The people he talked to where anywhere from ordinary unlettered men like fishermen to the jewish leaders of the day.
He spent a lot of time especially teaching the apostles. Jesus knew he didn't have much time left on earth. He was setting the platform for a christian congregation that could function much the same way in the manner of preaching gods word and teaching its meaning.
Fast-forward to today. We to continue Jesus' preaching work on earth. We teach gods intention for the earth Isaiah 45:18. We teach about Jehovah's qualities and commandments thus making disciples of all the nations Matthew 28:19-20. Jesus also assigned the apostles or leaders of the christian congregation at the time to care for the spiritual needs of the congregation Matthew 24:45.
We can't see god, but we can follow his laws and commandments. We can also learn a lot about gods qualities through his action in the past and what he has promised for the future.