This sermon by John Macarthur is almost comically bad from both an exegetical and historical standpoint. But for just a brief moment, let us focus on another problem--the astounding arrogance. https://www.gty.org/library/sermons-library/80-369/is-infant-baptism-biblical

After attributing a Reformed (of the baby-baptizing kind) understanding of infant baptism to Luther, and after also concluding that Luther "knew [infant baptism] wasn't in the Bible and wasn’t really right, Mac says this:

"The great mark of the Reformation was justification - justification - not by sacrament, not by ceremony, not by symbol. Justification by faith through grace...

How could [Reformers like Luther] understand that and then come up with something like infant baptism which by a rite on a baby confers to that baby salvation?"

I am happy to answer it for John MacArthur:

Luther believed in both sola fide and infant baptism because he was a better and humbler theologians than you.

Luther taught infant baptism because he was a strong enough theologian to realize that the Bible doesn't contradict itself, that the Bible said "salvation is through faith alone" and "baptism saves," and therefore that these statements were both true.

Luther was a good enough theologian to realize that faith receives the gifts that God puts into baptism. He was smart enough to see that, everywhere the Bible describes what baptism DOES, it describes baptism as God's work in us and for us.

Likewise, when it came to all the church fathers who believed in infant baptism and sola fide, Luther was humble enough to recognize that they weren't a bunch of idiots who couldn't put 2 and 2 together.

He was humble enough to recognize that, when doctrines may seem to contradict on the surface, if you actually listen to those who placed their hands into the scriptural dirt and dug a bit deeper, you would see what they saw.

Luther was humble enough to not say, "golly gee, I done red my bible for 15 seconds and I ddn't see a verses saying you have to baptise babys so I guess everybudy before me was not as smart as me."

In, summary, the reason Martin Luther, the greatest "justification by grace alone" articulator since the Apostle Paul also taught infant baptism was because he was a better and humbler theologian than John MacArthur.