1 Corinthians 9:27 but I punish my body and enslave it, so that after proclaiming to others I myself should not be disqualified.
2 Timothy 2:12 if we endure, we will also reign with him; if we deny him, he will also deny us;
Philippians 2:12 Therefore, my beloved, just as you have always obeyed me, not only in my presence, but much more now in my absence, work out your own salvation with fear and trembling; 13 for it is God who is at work in you, enabling you both to will and to work for his good pleasure.
13In Him, you also, after listening to the message of truth, the gospel of your salvation—having also believed, you were sealed in Him with the Holy Spirit of promise, 14who is given as a pledge of our inheritance, with a view to the redemption of God’s own possession, to the praise of His glory. Eph 1:13-14 NASB
21Now He who establishes us with you in Christ and anointed us is God, 22who also sealed us and gave us the Spirit in our hearts as a pledge. 2 Corinthians 1:21-22 NASB
Notice in both of these passages it is God who is the One doing the sealing. No where in the NT is the believer ever said to be unsealed nor is it ever said the believer can unseal what God has sealed.
Let's put 2 Tim 2:12 in context shall we:
11It is a trustworthy statement: For if we died with Him, we will also live with Him; 12If we endure, we will also reign with Him; If we deny Him, He also will deny us; 13If we are faithless, He remains faithful, for He cannot deny Himself. 2 Timothy 2:11-13 NASB
Paul concludes his letter to Timothy:
6For I am already being poured out as a drink offering, and the time of my departure has come. 7I have fought the good fight, I have finished the course, I have kept the faith; 8in the future there is laid up for me the crown of righteousness, which the Lord, the righteous Judge, will award to me on that day; and not only to me, but also to all who have loved His appearing.
Paul did not believe or teach one loses one's salvation.
An examination of the Greek behind the NT supports that conclusion.