James has made some important and weighty points in James 1:1-5:11, but now he says "above all." James stresses the importance of godliness. His use of the term "swear" which refers to vows. See "Honesty, Oaths, and Vows: Should I Swear?" and "Jephthah and Sinful Vows" below.
James' teaching obviously derived from Jesus' instruction on the subject of vows (Matt. 5:33-37). Neither Jesus nor James condemned proper vows and oaths. Rather, both condemned frivolous vows (cf. 2 Cor. 1:23; 1 Thess. 2:10). James was especially concerned that empty oaths would destroy relationships among believers.
To emphasize refraining from false oaths and vows so strongly is consistent with the Biblical concerns for covenant keeping and the for maintaining the sanctity of the faith. For more discussion of the proper role of oaths in the Christian life, see WCF 21.5; 22.1-5.
The Christian is expected to be a person whose word is reliable (Let your "Yes" be yes, and your "No," no). See WCF 22.2; HC 99, 102.
Drawing yet again from Jesus’ teaching in the Sermon on the Mount (see Matt. 5:33-37), James warns against swearing oaths by heaven, God’s throne, or the earth, its footstool in order to avoid condemnation. Oaths are not inherently sinful, because God binds himself to oaths (Acts 2:30; Heb. 3:11, 18; 4:3; 6:13, 16; 7:21; Rev. 10:6). But, oaths should not be frivolous and are less necessary in communities where a person’s yes means yes and no means no. See WCF 21.5, 22.2; a 99, 102.