John Gill Commentary Mark 4:32

"yet when it is sown, groweth up, and becometh greater than all the herbs, and putteth out great branches; so that the birds of the heaven can lodge under the shadow thereof." — Mark 4:32 (ASV)

But when it is sown, it grows up
So the Gospel, when it was preached, it spread notwithstanding all the opposition made against it by the Jews and Gentiles: there was no stopping it; though the Jewish sanhedrim charged the apostles to speak no more in the name of Jesus, they regarded them not; though Herod stretched forth his hands against the church, and killed one apostle, and put another in prison, yet "the word of God grew and multiplied", (Acts 12:1–3Acts 12:24) , and Gospel churches when set up, whether in Judea, or among the Gentiles, presently had additions made unto them, and "grew up", as holy temples in the Lord: and wherever the grace of God is really implanted, there is a growing in it, and in the knowledge of Christ Jesus:

and becomes greater than all herbs :
the Gospel exceeds the traditions of the Jews, and the philosophy of the Gentiles, and any human scheme whatever, in its nature, usefulness, and the largeness of its spread: and the Gospel church state will before long fill the world, and all nations shall flow unto it; when the Jews shall be converted, and the fulness of the Gentiles shall come, it will be a greater kingdom, than any of the kingdoms of the earth ever were: and the grace of God in the heart, is vastly above nature, and does that which nature can never perform; and which spreads and enlarges, and at last issues in eternal glory:

and shoots out great branches, so that the fowls of the air may
lodge under the shadow of it :
by whom are meant, saints; such to whom the Gospel is come in power, and who have the grace of God wrought in their hearts, who are partakers of the heavenly calling: these come where the Gospel is preached, and where gracious souls are met together, even in the several Gospel churches; where they not only come and go, but where they lodge, abide, and continue, under the shadow of the Gospel, and Gospel ordinances, and that with great delight and pleasure; singing songs of praise to God, for his electing and redeeming love, and for calling grace, and for all spiritual blessings, and Gospel privileges: for a larger explanation and illustration of this parable, (See Gill on Matthew 13:31), (See Gill on Matthew 13:32).