Through sanctification of the Spirit and belief of the truth !
2 Thess 2:13
13 But we are bound to give thanks alway to God for you, brethren beloved of the Lord, because God hath from the beginning chosen you to salvation through sanctification of the Spirit and belief of the truth:
No one can believe the Truth of God without first being partakers of this Work of the Spirit, for thats His main Purpose for being sent into the World for Christ's Sake ! For its by His Work here that all for whom Christ died are given a Saving Knowledge of the Truth, that is, for all God does desire to come into the Knowledge of the Truth as stated here 1 Tim 2:4
4 Who will have all men to be saved, and to come unto the knowledge of the truth.
The Holy Spirit answers God's desire and will here to bring all these men to the Truth.
By this [Sanctifying] Work of the Spirit in them, they are given Spiritual discernment to embrace the Things of God, the Truth of God 1 Cor 2:12-14
12 Now we have received, not the spirit of the world, but the spirit which is of God; that we might know the things that are freely given to us of God.[Which things the Truth set forth]
13 Which things also we speak, not in the words which man's wisdom teacheth, but which the Holy Ghost teacheth; comparing spiritual things with spiritual.
14 But the natural man receiveth not the things of the Spirit of God: for they are foolishness unto him: neither can he know them, because they are spiritually discerned.
You see that ? The Knowledge of the Truth to save us cannot be known apart from the Spirit's Work, because they are things spiritually discerned.
He is then to them [all whom Christ died] a Spirit of Wisdom as per Eph 1:17-18 58
17 That the God of our Lord Jesus Christ, the Father of glory, may give unto you the spirit of wisdom and revelation in the knowledge of him:
18 The eyes of your understanding being enlightened; that ye may know what is the hope of his calling, and what the riches of the glory of his inheritance in the saints,
A Hope called to by the Gospel ! Col 1:5
For the hope which is laid up for you in heaven, whereof ye heard before in the word of the truth of the gospel;