Non-Christains believe that we are wrong with equal fervor we believe they are. Do we have absolute proof that our is the right faith? No, we only believe that it is.
The OT wasn't wrong. Properly interpreted it teaches that God is to be honored on the Sabbath. Doing good honors God.
No, FK, the OT says that any work is prohibited on a Sabbath. It could't be more plain.
[Exodus 31:14] Therefore you are to observe the sabbath, for it is holy to you. Everyone who profanes it shall surely be put to death; for whoever does any work on it, that person shall be cut off from among his people
[Exodus 31:15, Lev 23:3] For six days work may be done, but on the seventh day there is a sabbath of complete rest, holy to the LORD; whoever does any work on the sabbath day shall surely be put to death.
[Jeremiah 17:22] You shall not bring a load out of your houses on the sabbath day nor do any work, but keep the sabbath day holy, as I commanded your forefathers.
[Jer 17:24] ...but to keep the sabbath day holy by doing no work on it
These are not interpretations, FK. This is supposedly straight from the words of God, recorded by Moses and prophets.
And I am telling you that Jesus discredited Moses and the prophets when He said that good works are acceptable on a Sabbath, because that's surely NOT what the OT says even thought that's what God may have had in mind, and the good authors of OT books just didn't "hear" it right! But there is no doubt as to what they wrote.
It seems that on the one hand the Orthodox position is that we can know nothing of the essence of God, but on the other hand we CAN KNOW that His essence is both mono-faceted AND definable as "love"???
Love is not God's essence; Love is God. And Love is never hate, lest it ceases being Love. You are human; at no instance can you be not human. God is God and He is never non-God. That much we can know (through negation).
No, FK, the OT says that any work is prohibited on a Sabbath. It couldn't be more plain. [several supporting scriptures]
It is obvious then that Jesus did not consider "doing good" to be "OT work", as in the passages you cite.