The Trinity
This trips many people up because it genuinely doesn't make sense, but ask yourself on an axiomatic level if something can be true without being comprehensible to the human mind. If something can't be true if it doesn't make sense to you, that means you think you are intelligent enough that your intellect can exhaust all of reality, but the human mind is limited and fallible. So there is a difference between asking "does scripture teach this" and "is this the way of interpreting scripture that makes sense". Sometimes the way that makes sense is a way that distorts the higher divine truth being revealed in scripture, which gives us a limited glimpse into something beyond human understanding. To deny the Trinity because some heresy is easier to understand is to forget how far above human comprehension God really is.
You become like what you worship
This connects deeply to the previous point, because you must worship the Trinity to be saved (I will clarify foreseen objections in the next point). Only the Trinitarian God is the perfect epitome of love, and that is why all societies that don't run on a Christian ethos are morally inferior. What you believe to be at the core of reality shapes and is shaped by your attitude towards the world, which shapes how you think and behave. The Trinity means that God's personality is characterized by being a loving father, a brother who would lay his life down for us, and a guiding light who is always with us all at the same time. It is important that this isn't viewed in a modalist way because only a God who is inherently INTERPERSONAL from eternity can have values that are inherently outward-facing rather than inward facing, and its important that it isn't viewed in a partialist way because that would undermine the perfect unity and intimacy of God's inherently loving and interpersonal nature.
"But the OT Jews/early church didn't always worship the Trinity"
They didn't worship a God other than the Trinitarian God either, they worshipped Yahweh as he was revealed to them to the best of their ability. Yahweh has revealed himself more fully as Trinitarian and so we must accept him as Trinitarian regardless of what our human minds prefer to think. We no longer have an excuse because wise and holy men have articulated the teaching of scripture in clear formulas such as the Athanasian Creed, which the heretics object to not because it goes against scripture but because it goes against what makes sense to them, so humble yourself and learn the difference. If you are an infant and get saved despite not intellectually assenting to something like the Trinity, it is because you did not reject grace by actively conceiving of something OTHER than the Trinity. Adults that aren't severely mentally disabled on the other hand will always be able to think of something other than God to put above God, in the case of heretics it is what they imagine God to be if he was limited by human reason which becomes their idol, and by actively refusing to worship God as revealed they reject his grace.
epistemology and interpretation
Sola Scriptura relies fundamentally on scripture providing its own hermeneutics, otherwise you would be better off as a cath or ortho than prot. Every single denomination claims to have the Holy Spirit guiding it's interpretation, don't be deceived by your own arrogance, if you interpret scripture too loosely or bring your own assumptions into it you cannot know that you are more protected by the HS from being wrong than the denomination or theologian that disagrees with you. That is why the true meaning of scripture must be derived solely from the text itself, the genres being employed and the historical context thereof and not from your own opinion. When you do that you realize mainstream christianity is right about the Trinity, scripture teaches something beyond human comprehension and all trinitarian heresies superimpose on scripture various forms of patchwork to change the meaning into something more palatable to human reason. Yet ironically, all these attempts to render it more understandable subtract from the beauty of the Trinity, the most elegant and meaningful way to articulate it happens to be the paradoxical way because the implication of it reflects most clearly who God is as a being who's core ethos is love.
Sacraments
Now lets apply what we have learned about interpreting the Bible to baptism and the Lord's Supper. The Bible explicitly says baptism saves, Jesus also explicitly tells his disciples "this is my body/blood which is shed for the forgiveness of sins" when instituting the Lord's Supper. So the interpretation that requires the least mental gymnastics is that baptism is a supernatural event inseparable from how a person gets saved, and literally plays a causal role, and that when you consume the consecrated bread/wine Jesus's body and blood are "with" them. The bread itself doesn't have to transubstantiate because that imports a complex and assumed metaphysical framework and its literal enough if his body is just as much there as the bread is. Besides, isn't it so much more meaningful to have a baptism and a eucharist that actually does something, cleansing you of sins and providing spiritual nourishment and union with Christ, than to have a mere symbolic observance?
soteriology
This inevitably brings into question how we are saved, because surely it is more complicated than "baptism saves". It is clear that salvation is intimately tied to the forgiveness of sins, which the Bible clearly states is delivered by both baptism and Christ's body/blood, but how then is it possible that we are saved by faith alone and that there are people who receive the sacraments and aren't saved as well as people who may be saved without receiving them? It is because faith is primarily spiritual and not intellectual, faith is delivered by the Holy Spirit through the means that God provides and tells us of in scripture, which are the word and sacrament. Those who "receive the kingdom of heaven as a little child", or in other words do not resist grace but instead passively let the spirit work through the gospel and sacraments, will be supernaturally transformed by the Holy Spirit. Good works are necessary because the HS is not neutral, he will push you to improve morally and if you refuse you are resisting grace, but what makes someone go from not saved to saved is ultimately not an activity of man but a passive process of letting God sow the seed of the kingdom of heaven in your heart which happens when you hear the word and/or receive the sacraments.
free will
There is a false dichotomy between arminian protestantism and calvinist protestantism, both import their own assumptions into things. The Biblical position is that we have no power to save ourselves, yet God does not damn anyone just because he felt like it. If this sounds too paradoxical for you, you didn't learn your lesson from our discussion of the Trinity. Our will in spiritual matters is bound by sin, and we can only be freed from that bondage passively, it is impossible for man by his own efforts to come to faith. We do however have a significant degree of control over how stubbornly we resist grace, so it is simultaneously true that you have no power to choose salvation and that going to hell is your choice and not God's, before you are granted faith you can only be more or less open minded to it.