I am in the midst of leading a Bible study series for my cell group, and in one of the Bible study, the passages I brought up are the one by Witsius as seen here. Here therefore is the proposed Scriptural cross-references for the propositions in the quotes taken from his book The Economy of the Covenants:
... it must be confessed, that in the present dark state of our minds, even the most illuminated are ignorant of a great many things (Job. 38-40:5; Is. 55:8-9); and that many things are believed with an implicit [tacit] faith (Mt. 7:17,20; 17:20, cf. Jn. 9), especially by young beginners and babes in Christ, so far as they admit, in general, the whole scriptures to be the infallible standard of what is to be believed; in which are contained many things which they do not understand, and in as far as they embrace the leading doctrines of Christianity, in which many other truths concenter, which are thence deduced by evident consequence, and which they believe in their foundation or principle, as John writes concerning believers, that they knew all things, 1 John ii. 20. ... (III.7.9. Vol. 1, p. 376.)
Moreover those things which are necessary to be known by the person who would believe, are in general, the divinity of the scriptures (Ps. 119 esp. v. 81, 114; Eph. 1:13), into which faith must be ultimately resolved; more especially, those things which regard the obtaining of salvation in Christ; which may summarily be reduced to these three heads (Eph. 1:13): 1st To know, that by sin thou art estranged from the life of God, and art come short of the glory of God, Rom. iii.23. That it is not possible, that either thou thyself, or an angel from heaven, or any creature in the world, nay, or all the creatures in the universe, can extricate thee from the abyss of misery, and restore thee to a state of happiness. 2dly. That thou shouldst know Christ this Lord to be full of grace and truth, John i.14. who is that only name given under heaven, whereby we can be saved, Acs. iv. 1. and in the knowledge of whom consists eternal life, John xvii. 3. 3dly. That thou shouldst know, that, in order to thy obtaining salvation in Christ, it is necessary that thou be united to Christ, by the Spirit and by faith, and give up thyself to him, not only to be justified, but also sanctified, and governed by his will and pleasure, proving what is that good, and acceptable, and perfect will of God, Rom. xii. 2. (III.7.10. Vol. 1, p. 377.)
The three things that all Christians must hold to therefore are: 1) The divinity of the Scripture, 2) the message of the Gospel, 3) the divinity of Christ and the Godhead. These are the fundamental truths of Scripture which all Christians must hold to in order to be saved.