Romans 12:1 I beseech you therefore, brethren, by the mercies of God, that ye present your bodies a living sacrifice, holy, acceptable unto God, which is your reasonable service.
It's reasonable for all Christians to live their life serving God. God calls it a reasonable responsibility.
In Romans 12:5 we start to get an understanding of what it means to be a member of the church, which is Christ's body.
In Romans 12:6-8 we see a list of spiritual gifts. These spiritual gifts are given to believers by God to be used to serve Him and His body.
When you minister to someone, when you give, when you exhort, you are actively using your spiritual gifts to serve Christ and His Church.
Romans 12:6-8 Having then gifts differing according to the grace that is given to us, whether prophecy, let us prophesy according to the proportion of faith; Or ministry, let us wait on our ministering: or he that teacheth, on teaching; Or he that exhorteth, on exhortation: he that giveth, let him do it with simplicity; he that ruleth, with diligence; he that sheweth mercy, with cheerfulness.
The bottom line is: Christians are called to serve one another.
Galatians 5:13 For, brethren, ye have been called unto liberty; only use not liberty for an occasion to the flesh, but by love serve one another.
Christians are specially called to serve the living God.
Hebrews 9:14 How much more shall the blood of Christ, who through the eternal Spirit offered himself without spot to God, purge your conscience from dead works to serve the living God?
You're going to serve something.
Before someone is saved, they spend their life serving idols. After someone is saved, they turn from serving idols to serving the one, true and living God.
1 Thessalonians 1:9 For they themselves shew of us what manner of entering in we had unto you, and how ye turned to God from idols to serve the living and true God;