The word "legalism" cannot be found in Scripture, & is therefore a modern term. The argument that legalism only refers to adding the Law to salvation is fallacious at best. Since it is an English word, we must use an English dictionary to discover its definition; not proof-text it.
The Merriam-Webster dictionary defines legalism as "strict, literal, or excessive conformity to the law or to a religious or moral code <the institutionalized legalism that restricts free choice>"
I'd say that is an excellent definition that describes the many IFB churches to a "T". Legalism ALWAYS restricts free choice because it is a means of controlling others via unScriptural means. This is also a very good definition of the leaven of the Pharisees. Hence, legalistic churches are viewed as modern Pharisees.
The word trinity is not found in the Bible either, but the concept is. Neither are the words: theology, Christology, Bibliology, etc. But I do hope you believe in the study of each and every one of them. There are many words that we used to define concepts that are not in the Bible, but are clearly taught in the Bible. Here is where legalism is clearly taught in the Bible:
And certain men which came down from Judaea taught the brethren, and said,
Except ye be circumcised after the manner of Moses, ye cannot be saved. (Acts 15:1)
--These were the first legalists. They followed Paul wherever he went trying to disrupt the work that he started. Most of Galatians deals with this problem. What did these men do and believe. They added their works (circumcision and the keeping of the law) as requirements to salvation. That is legalism. Whenever you add to the gospel message of grace, works, then it is legalism. The Church of Christ does that.
You can find many definitions of the word "church" in an English dictionary. But only one fits the Bible's definition. We go to the Bible, not English dictionaries for our theology.