CathInfo.com

Some theologians hold that view at least in regard to those who lived in pagan lands where the Gospel had not yet been preached.

St. Alphonsus held it as the "more probable" and "more common" opinion that belief in the Trinity is required by all by necessity of means for salvation.

However, he explains all three opinions, i.e.

1) that explicit belief in the Trinity is "necessitate medii" for all;

2) that explicit belief is "necessitate medii" for all but in some rare cases God makes exception to this;

3) that explicit belief in the Trinity is only "necessitate praecepti" and that only implicit belief in the Trinity is required "necessitate medii."

He calls this third opinion (which he lists as the second) "also probable enough" and he quotes a number of eminent theologians who hold this opinion. Here is the passage in his works:

https://books.google.com/books?id=NR48AAAAMAAJ&pg=PA296

Now they (adults) are disposed unto the said justice, when, excited and assisted by divine grace, conceiving faith by hearing, they are freely moved towards God, believing those things to be true which God has revealed and promised, and this especially, that God justifies the impious by His grace, through the redemption that is in Christ Jesus; and when, understanding themselves to be sinners, they, by turning themselves, from the fear of divine justice whereby they are profitably agitated, to consider the mercy of God, are raised unto hope, confiding that God will be propitious to them for Christ's sake; and they begin to love Him as the fountain of all justice; and are therefore moved against sins by a certain hatred and detestation, to wit, by that penitence which must be performed before baptism: lastly, when they purpose to receive baptism, to begin a new life, and to keep the commandments of God. Concerning this disposition it is written; He that cometh to God, [1] must believe that he is, and [2] is a rewarder to them that seek him; [even Trent only mentions two necessities, why didn't they check with the Church of Ladislaus first?] and, Be of good faith, son, thy sins are forgiven thee; and, The fear of the Lord driveth out sin; and, Do penance, and be baptized every one of you in the name of Jesus Christ, for the remission of your sins, and you shall receive the gift of the Holy Ghost; and, Going, therefore, teach ye all nations, baptizing them in the name of the Father, and of the Son, and of the Holy Ghost; finally, Prepare your hearts unto the Lord.

All of the supernatural aids granted by God to any man tend to lead him to the eternal possession of the Beatific Vision. They likewise direct him toward those realities which, either by their very nature or by God's own institution, are requisite for the attainment of the Beatific Vision. One of those realities is the visible Catholic Church, the religious society over which the Bishop of Rome presides as the Vicar of Christ on earth. The graces which God grants to any man outside the Church will inevitably guide him in the direction of the Church. Fenton