Here Paul says to Gentile believers: "Behold then the kindness and severity of God; to those who fell, severity, but to you, God's kindness, if you continue in His kindness; otherwise you also will be cut off." Does this imply that genuine believers can lose their salvation? I don't think so.
First of all, it may be that Paul is echoing a theme found elsewhere in his letters and throughout the NT, namely, that ultimate salvation is dependent on perseverance in faith (cf. Rom. 8:13; Col. 1:23; Heb. 3:6,14; 1 Peter 1:5; 1 John 2:19), a faith which God graciously preserves and sustains within us.
Second, others have suggested that Paul's discussion here is about Gentiles as a class, considered collectively, and Israel as a class, considered collectively. In other words, just as "Israel" was cut off, so also "Gentiles" may be if they do not believe.
Third, in his commentary on Romans, Douglas Moo provides this explanation: "While the olive tree represents the true, spiritual people of God, those who are said to belong to this tree are not only those who, through their faith, are actually part of the tree but also those who only appear to belong to that tree. This is evident from the fact that Paul speaks of unbelieving Jews as having been 'cut off' from the tree (v. 17). In reality, these Jews had never been part of the tree at all; yet to preserve the metaphor he is using, Paul presents them as if they had been. In the same way, then, those Gentiles within the church at Rome 'and elsewhere' who appear to be part of God's people, yet do not continue in faith, may never have been part of that tree at all" (707).
Views (1) and (3) may then be combined: Those who have truly believed will continue in God's kindness (Heb. 3:6,14). Those who do not continue in God's kindness show thereby that they were never truly part of the tree (on this, see esp. 1 John 2:19). Failure to persevere does not mean that one who was truly saved becomes truly lost. Rather, perseverance is itself the proof that one was truly saved. If one does not persevere, one has always been lost and never saved.