Oecumenius

I am coming quickly, he says, having taken hold of you. Hold fast what you have, that no one may take your crown. And what do you hold fast? The genuine love toward the Lord, evidently, in which, having persevered until the end, you will receive the crown of life (James 1:12). For to those who endure, belong the victories.

Commentary on Revelation ↗

Cyprian

For confession [of Christ] does not make one immune from the snares of the devil. Nor does it defend one who is still placed in the world with a perpetual security against worldly temptations and dangers and onsets and attacks. Otherwise we should never have seen afterwards among the confessors the deceptions and debaucheries and adulteries that now with groaning and sorrow we see among some. Whoever that confessor is, he is not greater or better or dearer to God than Solomon. …

Treatise I. On the Unity of the Church 20 ↗

Bede

Behold, I come quickly. Hold fast what you have, etc. Lest you grow weary in enduring. For I will quickly help, lest perhaps, if you fail, another receives the reward decreed for you. Thus, the number of saints, which is fixed with God, cannot be shortened by the treachery of growing tares. If indeed a crown is given to another when lost, the place of him who lost what he held is not vacant.

Commentary on Revelation ↗

Alcuin of York

I come quickly: hold fast that which thou hast. The Lord says he will come quickly, so that no one should give up in the struggle. For everything that passes happens quickly. What he tells him to hold fast is the conduct of heavenly life, and he gives the reason, adding, that no other man take thy crown. In this phrase we see that the certain and determined number of the elect is embraced, and we contemplate with an absolutely amazing thought the fact that no one can be added to or subtracted from it.

Commentary On Revelation ↗