I wish the A, B and C of the matrices defined as follow were written using the default computer modern font. I'm using the fontspec package with LuaLaTeX, but when I set the main font it also changes the math bold font. Here's an excerpt of my document:
\documentclass[12pt,a4paper]{report}
\usepackage{polyglossia}
\setdefaultlanguage{english}
\disablehyphenation
\usepackage[
hmarginratio = 1:1,
left = 3cm,
right = 2cm,
top = 3cm,
bottom = 2cm
]{geometry}
\usepackage{fontspec}
\setmainfont{Arno Pro}
\usepackage{amsmath}
\usepackage{lipsum}
\begin{document}
\lipsum[1]
\begin{center}
$ {\bf A}_{2\times 3}=\begin{pmatrix}
1 & 2 & 3 \\
4 & 5 & 6
\end{pmatrix}
%
\hspace{.5cm}
%
{\bf B}_{4\times 3}=\begin{bmatrix}
1 & 2 & 3 \\
4 & 5 & 6 \\
7 & 8 & 9 \\
1 & 2 & 3
\end{bmatrix}
%
\hspace{.5cm}
%
{\bf C}_{2\times 2}=\begin{Vmatrix}
1 & 2 \\
3 & 4
\end{Vmatrix} $
\end{center}
\lipsum[2]
\end{document}
As you can see the bold math font is not the computer modern but the numbers are. Although both of them are inside the math environment.

\bf. See https://tex.stackexchange.com/q/516/35864 and https://tex.stackexchange.com/q/15361/35864.\mathbfseems more appropriate here. But I'm not sure if this solves your problem. But even if not, you should definitely not be using\bfany more anywhere. – moewe May 31 '18 at 13:53See the LaTeX manual or LaTeX Companion for explanation. Type H for immediate help.
...
l.16 \setmainfont {Arno Pro} The package fontspec has already been loaded with options: [] There has now been an attempt to load it with options [no-math] Adding the global options: ,no-math to your \documentclass declaration may fix this. Try typing to proceed.
– Levy May 31 '18 at 13:58@moeweI changed to\textbf{}and it didn't solve the problem. – Levy May 31 '18 at 14:00\mathbfand not\textbf. And that is what I would use. But I don't know if this will solve the actual issue at hand. Still you should not be using\bf. – moewe May 31 '18 at 14:02fontspecbeforepolyglossia. – moewe May 31 '18 at 14:02@moeweI'm sorry. I meant\mathbfnot\textbf. Also, I loadedfontspecbeforepolyglossia, but the issue continues. – Levy May 31 '18 at 14:04Linux Libertineinstead ofArno Pro, but that should be irrelevant) if I load\usepackage[no-math]{fontspec}beforepolyglossiaand use\mathbf. – moewe May 31 '18 at 14:06fontspecbeforepolyglossiaand added the option[no-math]. Thanks to@moeweand@Ulrike Fischeranswers combined it's all nice and tidy. – Levy May 31 '18 at 14:08