[HttpPost]
public ActionResult Login(Models.Logins user)
{
    if (ModelState.IsValid)
    {
        if (user.IsValid(user.Username, user.Password))
        {
            FormsAuthentication.SetAuthCookie(user.Username, user.RememberMe);
            return RedirectToAction("Index", "Home");
        }
        else
        {
            ModelState.AddModelError("", "Login gegevens kloppen niet!");
        }
    }
    return View(user);
}

using System;
using System.Collections.Generic;
using System.ComponentModel.DataAnnotations;
using System.Data;
using System.Data.Entity;
using System.Data.SqlClient;
using System.Linq;
using System.Web;

namespace RTApplicatie.Models
{
    public class Logins
    {
        [Required]
        public int ID { get; set; }

        [Required]
        [Display(Name = "User name")]
        public string Username { get; set; }

        [Required]
        [DataType(DataType.Password)]
        [Display(Name = "Password")]
        public string Password { get; set; }

        [Display(Name = "Remember on this computer")]
        public bool RememberMe { get; set; }

        [Required]
        public bool Rights { get; set; }
        /// <summary>
        /// Checks if user with given password exists in the database
        /// </summary>
        /// <param name="_username">User name</param>
        /// <param name="_password">User password</param>
        /// <returns>True if user exist and password is correct</returns>
        public bool IsValid(string _username, string _password)
        {
            using (var cn = new SqlConnection(@"Data Source=(localdb)MSSQLLocalDB;"))
            {
                string _sql = @"SELECT [Username] FROM [dbo].[System_Users] " +
                       @"WHERE [Username] = @u AND [Password] = @p";
                var cmd = new SqlCommand(_sql, cn);
                cmd.Parameters
                    .Add(new SqlParameter("@u", SqlDbType.NVarChar))
                    .Value = _username;
                cmd.Parameters
                    .Add(new SqlParameter("@p", SqlDbType.NVarChar))
                    .Value = Helpers.SHA1.Encode(_password);
                cn.Open();
                var reader = cmd.ExecuteReader();
                if (reader.HasRows)
                {
                    reader.Dispose();
                    cmd.Dispose();
                    return true;
                }
                else
                {
                    reader.Dispose();
                    cmd.Dispose();
                    return false;
                }
            }
        }
    }

    public class RekenContext : DbContext
    {
        public DbSet<Logins> RTApplicatie { get; set; }
    }
}