Don’t say you believe something when you don’t obey it!
All through the Bible believing is equated with obeying!
The gospel of Jesus Christ must be believed. The gospel of Jesus Christ must be obeyed!  We must believe in Jesus Christ as the Saviour of the world. Although the gospel is an invitation to believe in Jesus, it is also a command to obey Jesus.  To believe in the gospel is to pledge yourself to obey the commandments of God.

When you receive the gospel it is not just to give a mental assent to the existence of Jesus Christ.   Believing the gospel is to obey the gospel of Jesus Christ.   Notice that the scriptures below teach us that receiving and believing in the gospel is the same as obeying the gospel. All through the Bible believing is equated with obeying!
1. Believing in the gospel is seen as obeying the gospel.
For the time is come that judgment must begin at the house of God: and if it first begin at us, what shall the end be of them that obey not the gospel of God?
1 Peter 4:17

2. Believing in the gospel and obeying the gospel is one and the same thing.
And to you who are troubled rest with us, when the Lord Jesus shall be revealed from heaven with his mighty angels, In flaming fire taking vengeance on them that know not God, and that obey not the gospel of our Lord Jesus Christ: Who shall be punished with everlasting destruction from the presence of the Lord, and from the glory of his power;
2 Thessalonians 1:7-9

3. Obeying the gospel is all that God expects from us.  
But they have not all obeyed the gospel. For Esaias saith, Lord, who hath believed our report? So then faith cometh by hearing, and hearing by the word of God.
Romans 10:16-17