Yes (should be approached with caution), we can consider Adventism as part of Christianity rather than Kingdom of the Cults or outside Christianity. The Adventist community is a diverse movement… But there is a clear confusion when you talk to an Adventist regarding the unity in the Body of Christ. Why is there an exclusivity? Jesus prayed for the unity of his followers, the apostles also preached unity in the Body of Christ. This is also applicable to other denominations who are exclusive in their theology and mark themselves as the only true church of Jesus Christ.
According to the Scriptures:
John 17:21 NLT I pray that they will all be one, just as you and I are one—as you are in me, Father, and I am in you. And may they be in us so that the world will believe you sent me. ~Jesus Christ prayed for his followers
1 Corinthians 1:10 NLT I appeal to you, dear brothers and sisters, by the authority of our Lord Jesus Christ, to live in harmony with each other. Let there be no divisions in the church. Rather, be of one mind, united in thought and purpose. ~Paul
1 Corinthians 1:30 NLT God has united you with Christ Jesus. For our benefit God made him to be wisdom itself. Christ made us right with God; he made us pure and holy, and he freed us from sin. ~Paul
1 Corinthians 12:12-13 NLT The human body has many parts, but the many parts make up one whole body. So it is with the body of Christ. 13 Some of us are Jews, some are Gentiles, some are slaves, and some are free. But we have all been baptized into one body by one Spirit, and we all share the same Spirit. ~Paul
Galatians 3:26-28 NLT For you are all children of God through faith in Christ Jesus. 27 And all who have been united with Christ in baptism have put on Christ, like putting on new clothes. 28 There is no longer Jew or Gentile, slave or free, male and female. For you are all one in Christ Jesus. ~Paul
Ephesians 4:4-6 NLT For there is one body and one Spirit, just as you have been called to one glorious hope for the future. 5 There is one Lord, one faith, one baptism, 6 one God and Father of all, who is over all, in all, and living through all. ~Paul
Philippians 1:27 NLT Above all, you must live as citizens of heaven, conducting yourselves in a manner worthy of the Good News about Christ. Then, whether I come and see you again or only hear about you, I will know that you are standing together with one spirit and one purpose, fighting together for the faith, which is the Good News. ~Paul
Philippians 2:2 NLT Then make me truly happy by agreeing wholeheartedly with each other, loving one another, and working together with one mind and purpose. ~Paul
1 Peter 3:8 NLT Finally, all of you should be of one mind. Sympathize with each other. Love each other as brothers and sisters. Be tenderhearted, and keep a humble attitude. ~Peter
See also: Case on Seventh-day Adventist