Reasons 1–3: Scripture's own witness; internal lamp; Christ's promise of the Spirit
The reasons are these.
1. From Scripture's own testimony (Deuteronomy 30:11–Deuteronomy 30:14): that place does not speak of the ease of keeping the law, but of the ease of understanding it — which, with Alfonso Tostado Bishop of Avila, Bellarmine himself (On grace and free will book 3 chapter 6) proves and confesses to seem the most literal and proper reading. And what Moses said here of Old Testament doctrine Paul applies to the dogmas of the New Testament (Romans 10:6 and following).
2. From Scripture's inward property, by which it is called a lamp to our feet (Psalm 119:105), a lamp shining in a dark place (2 Peter 1:19); add Psalm 19:4–Psalm 19:9, Psalm 19:12.
3. From Christ's promise (John 16:12-John 16:13): "I still have many things to say to you, but you cannot bear them now. When he, the Spirit of truth, comes, he will lead you into all the truth; for he will not speak from himself, but whatever he hears he will speak, and he will announce to you the things that are to come." Likewise 1 John 2:27: "The anointing teaches you concerning all things, and is true and is not a lie; and as it taught you, you will remain in him." By this it is signified that we are most fully instructed in the things needful to salvation — which is wrongly dragged to bishops gathered in a council as if they were infallible, of whom Bellarmine (On the authority of councils book 2 chapter 2) confesses that, existing apart, they are not taught every truth by the Holy Spirit.
Latin
Rationes sunt istae, 1. Ab ipsius scripturae testimonio, Deut. 30.v.11.12.13.14. qui locus non loquitur de facilitate observandae legis, sed de facilitate intelligendae: id quod cum Alphonso Tostato episcopo Abulensi ipse Bellarminus lib. 3. de gratia & libero arbitrio cap. 6. probat & fatetur hanc interpretationem maxime literalem & propriam videri. Quae vero hic a Mose de doctrina veteris Testamenti dicta sunt, Paulus dogmatibus novi Testamenti accommodat, Rom. 10. v.6. & seqq. 2. Ab interna scripturae proprietate, qua dicitur lucerna pedibus nostris, psal. 119. v. 105. lucerna splendens in obscuro loco, 2. Petr. 1.v.19. adde psal. 19. 4.5.6.7.8.9.12. 3. A promissione Christi, Joh. 16.12.13. Adhuc multa habeo quae vobis dicam, sed nunc non potestis portare. Quum autem venerit ille, Spiritus veritatis, deducet vos in omnem veritatem. non enim loquetur a semetipso, sed quaecunque audierit loquetur, & quae ventura sunt, renunciabit vobis. Sic 1. Joh. 2.27. Unctio docet vos de omnibus, & verax est & non mendax: & sicut docuit vos, manebitis in eo. quo significatur nos esse instructissimos a rebus ad salutem necessariis, quod perperam trahitur ad episcopos in concilio congregatos, quasi sint infallibiles, de quibus Bellarminus fatetur lib.2. de concil. auct. cap. 2. seorsim existentes non doceri a Spiritu sancto omnem veritatem.