The church at Thyatira was to actively hold onto the good things they had until the coming of Christ (Rev. Rev. 2:19+). They must be held fast in the face of active opposition by the flesh, the devil, and enemies of the church. This is the theme throughout Scripture for those who seek after God.
I have chosen the way of truth; Your judgments I have laid before me. I cling to Your testimonies; O LORD, do not put me to shame! I will run the course of Your commandments, for You shall enlarge my heart. Teach me, O LORD, the way of Your statutes, and I shall keep it to the end. Give me understanding, and I shall keep Your law; indeed, I shall observe it with my whole heart. Make me walk in the path of Your commandments, for I delight in it. (Ps. Ps. 119:30-35) [emphasis added]The Christian life is like a greased pole that we are either actively climbing up or passively sliding down. There is no opportunity to remain stationary.
until I come
The imminent coming of Jesus is an important theme throughout this book (Rev. Rev. 1:7+. Rev. 22:7+, Rev. 22:20+). That this coming is not a spiritual coming can be seen by the context. The promises of the next few verses are for he who overcomes until the end (Rev. Rev. 2:26+) and include being given authority at the commencement of the millennial reign (Rev. Rev. 20:4+). See Imminency. See Theme.
1 Here is an example of a verb in an aorist tense which implies continuous action.