If you hold to the fact that "your belief in the gospel" is the cause of your eternal salvation, then it is a work, a requirement for man to conform to in order to get eternal life. Eternal salvation is given by the grace of God, without the help of man.
Belief is not a work of the law. Why don't you understand this? No works of the law can save a man. Believing the gospel of Jesus Christ is not a work of the law.
Romans 10
1 Brethren, my heart's desire and prayer to God for Israel is, that they might be saved.
2 For I bear them record that they have a zeal of God, but not according to knowledge.
3 For they being ignorant of God's righteousness, and going about to establish their own righteousness
(through works of the law), have not submitted themselves unto the righteousness of God
(through belief in the gospel of Jesus Christ).
4
For Christ is the end of the law for righteousness to every one that believeth.
5 For Moses describeth the righteousness which is of the law, That the man which doeth those things shall live by them.
13 For whosoever shall call upon the name of the Lord shall be saved.
14 How then shall they call on him in whom they have not believed? and how shall they believe in him of whom they have not heard? and how shall they hear without a preacher?
15 And how shall they preach, except they be sent? as it is written, How beautiful are the feet of them that preach the gospel of peace, and bring glad tidings of good things!
16
But they have not all obeyed the gospel. For Esaias saith, Lord,
who hath believed our report?
17 So then faith cometh by hearing, and hearing by the word of God.