Heretics fell by the wayside BECAUSE they did not rightly divide "the word"...Even the ECF's stated as much, that it is clear in scripture and that others would fail trying to use scripture to support their beliefs.
and BTW the Trinity is certainly indirectly implied in the OT. Right in the very first chapter of Genesis as well as in Isaiah to name a few...You need not be some high level genius to figure it out, just rightly dividing scripture...
Gen 1:26 And God said, Let us make man in our image, after our likeness: and let them have dominion over the fish of the sea, and over the fowl of the air, and over the cattle, and over all the earth, and over every creeping thing that creepeth upon the earth.
27 So God created man in his own image, in the image of God created he him; male and female created he them.
Isa 9:6 For unto us a child is born, unto us a son is given: and the government shall be upon his shoulder: and his name shall be called Wonderful, Counsellor, The mighty God, The everlasting Father, The Prince of Peace.
Isa 63:8 For he said, Surely they are my people, children that will not lie: so he was their Saviour.
9 In all their affliction he was afflicted, and the angel of his presence saved them: in his love and in his pity he redeemed them; and he bare them, and carried them all the days of old.
10 But they rebelled, and vexed his holy Spirit: therefore he was turned to be their enemy, and he fought against them.