In a very real sense, it is both. The power is in the gospel itself.
Rom 1:16
For I am not ashamed of the gospel of Christ: for it is the power of God unto salvation to every one that believeth; to the Jew first, and also to the Greek.
However, Christians are called to persuade people to believe the gospel.
Rom 10:13
For whosoever shall call upon the name of the Lord shall be saved.
Rom 10:14
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?
Rom 10:15
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!
Rom 10:16
But they have not all obeyed the gospel. For Esaias saith, Lord, who hath believed our report?
Rom 10:17
So then faith
cometh by hearing, and hearing by the word of God.
Rom 10:18
But I say, Have they not heard? Yes verily, their sound went into all the earth, and their words unto the ends of the world.
Rom 10:19
But I say, Did not Israel know? First Moses saith, I will provoke you to jealousy by
them that are no people,
and by a foolish nation I will anger you.
Rom 10:20
But Esaias is very bold, and saith, I was found of them that sought me not; I was made manifest unto them that asked not after me.
Rom 10:21
But to Israel he saith, All day long I have stretched forth my hands unto a disobedient and gainsaying people.
Again, the gospel of peace is the glad tidings of good things, but Christians are called to have their feet shod with the preparation of the same.
Eph 6:15
And your feet shod with the preparation of the gospel of peace;
Anyhow, as Paul rightly said, faith comes by hearing, and hearing by the word of God. It is not something that God zaps people with against their own free will. Israel heard, but, sad to say, the gospel that they heard was not mixed with faith by most who heard it.
Heb 4:2
For unto us was the gospel preached, as well as unto them: but the word preached did not profit them, not being mixed with faith in them that heard
it.
Why wasn't it mixed with faith? Paul told us why. For the most part, the Israelites were a disobedient and gainsaying people who refused to hear. This is what the scriptures repeatedly teach, and not the nonsense that several of you here are spouting.