Saadia Gaon dealt with the two basic issues of Christianity of cancellation of the commandments and the Trinity.
But for some reason his book אמונות ודעות [Faiths and World Views] is rarely learned. The reason is that is is not on Gemara nor Musar. Still it sets the stage for all later Musar. [See the Introduction to the Obligations of the Hearts.]
In any case I wanted to mention that certainly he was correct that cancellation of the commandments is wrong. The trouble with this is it is not from Jesus himself but rather from Paul. And Paul was referring to gentiles. So while Saadia Gaon was right to criticize the doctrine, the point is that the doctrine is itself not even in the NT. note 1 [see http://www.anthonyflood.com/bahnsentheonomicposition.htm]
However, I ought to add that there is the point of R Shimon Ben Yochai-- that the commandments have a purpose. And when the purpose does not apply to a situation, the commandment itself is not applicable. For instance: it is not allowed to take a pledge from a rich widow. R. Shimon Ben Yochai said one can take a pledge from a rich widow. [BM 119] [See Rav Shach where he brings up this argument between R Shimon and the Sages.]
The Trinity is also apparently a difficulty except if one takes into account that all mystics generally held the souls of the patriarchs from from Emanation. [I mean all mystics. See Sefer Yezira, Sefer HaBahir, Rav Avraham Abulafia etc.]
[note 1] "Do not begin to think that I came to abrogate the Law or the Prophets; I came not to abrogate but to fulfill. For truly I say to you, until heaven and earth pass away, until all things have happened, not one jot or tittle shall by any means pass away from the law. Therefore, whoever shall break one of these least commandments and teach men so shall be called least in the kingdom of heaven" (Matt. 5:17-19).
But for some reason his book אמונות ודעות [Faiths and World Views] is rarely learned. The reason is that is is not on Gemara nor Musar. Still it sets the stage for all later Musar. [See the Introduction to the Obligations of the Hearts.]
In any case I wanted to mention that certainly he was correct that cancellation of the commandments is wrong. The trouble with this is it is not from Jesus himself but rather from Paul. And Paul was referring to gentiles. So while Saadia Gaon was right to criticize the doctrine, the point is that the doctrine is itself not even in the NT. note 1 [see http://www.anthonyflood.com/bahnsentheonomicposition.htm]
However, I ought to add that there is the point of R Shimon Ben Yochai-- that the commandments have a purpose. And when the purpose does not apply to a situation, the commandment itself is not applicable. For instance: it is not allowed to take a pledge from a rich widow. R. Shimon Ben Yochai said one can take a pledge from a rich widow. [BM 119] [See Rav Shach where he brings up this argument between R Shimon and the Sages.]
The Trinity is also apparently a difficulty except if one takes into account that all mystics generally held the souls of the patriarchs from from Emanation. [I mean all mystics. See Sefer Yezira, Sefer HaBahir, Rav Avraham Abulafia etc.]
[note 1] "Do not begin to think that I came to abrogate the Law or the Prophets; I came not to abrogate but to fulfill. For truly I say to you, until heaven and earth pass away, until all things have happened, not one jot or tittle shall by any means pass away from the law. Therefore, whoever shall break one of these least commandments and teach men so shall be called least in the kingdom of heaven" (Matt. 5:17-19).
Several points about the interpretation of this passage should be rather clear. (1) Christ twice denied that His advent had the purpose of abrogating the Old Testament commandments. (2) Until the expiration of the physical universe, not even a letter or stroke of the law will pass away. And (3) therefore God’s disapprobation rests upon anyone who teaches that even the least of the Old Testament laws may be broken.
Attempts are sometimes made to evade the thrust of this text by editing out its reference to the moral demands of the Old Testament—contrary to what is obvious from its context (5:16, 20, 21-48; 6:1, 10, 33; 7:12, 20-21, 26) and semantics (“the law” in v. 18, “commandment” in v. 19). Other attempts are made to extract an abrogating of the law’s moral demands from the word “fulfill” (v. 17) or the phrase “until all things have happened” (v. 18). This, however, renders the verses self-contradictory in what they assert.